internal/app/app.go

@@ -59,6 +59,7 @@ func New(cfg *config.Config) (*App, error) {
 	spaceService.SetAuditLogger(auditLogService)
 	accountService := service.NewAccountService(accountRepository)
 	accountService.SetAuditLogger(auditLogService)
+	accountService.SetAllocationRepository(allocationRepository)
 	allocationService := service.NewAllocationService(allocationRepository, accountService)
 	allocationService.SetAuditLogger(auditLogService)
 	transactionService := service.NewTransactionService(transactionRepository, categoryRepository, accountService)

internal/db/migrations/00002_create_workspace_collaboration_tables.sql

@@ -34,5 +34,5 @@ CREATE TABLE space_invitations (
 -- +goose StatementBegin
 DROP TABLE space_invitations;
 DROP TABLE space_members;
-DROP TABLE spaces;
+DROP TABLE spaces CASCADE;
 -- +goose StatementEnd

internal/db/migrations/00003_create_financial_management_tables.sql

@@ -40,5 +40,5 @@ CREATE TABLE transaction_tags (
 DROP TABLE transaction_tags;
 DROP TABLE tags;
 DROP TABLE transactions;
-DROP TABLE accounts;
+DROP TABLE accounts CASCADE;
 -- +goose StatementEnd

internal/db/migrations/00014_add_currency_to_accounts.sql

@@ -0,0 +1,9 @@
+-- +goose Up
+-- +goose StatementBegin
+ALTER TABLE accounts ADD COLUMN currency TEXT NOT NULL DEFAULT 'CAD';
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+ALTER TABLE accounts DROP COLUMN currency;
+-- +goose StatementEnd

internal/handler/space.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"git.juancwu.dev/juancwu/budgit/internal/ctxkeys"
+	"git.juancwu.dev/juancwu/budgit/internal/misc/currency"
 	"git.juancwu.dev/juancwu/budgit/internal/model"
 	"git.juancwu.dev/juancwu/budgit/internal/routeurl"
 	"git.juancwu.dev/juancwu/budgit/internal/service"
@@ -221,6 +222,7 @@ func (h *spaceHandler) SpaceOverviewPage(w http.ResponseWriter, r *http.Request)
 			ID:       a.ID,
 			Name:     a.Name,
 			Balance:  a.Balance,
+			Currency: a.Currency,
 		})
 	}
 
@@ -253,14 +255,27 @@ func (h *spaceHandler) SpaceCreateAccountPage(w http.ResponseWriter, r *http.Req
 func (h *spaceHandler) HandleCreateAccount(w http.ResponseWriter, r *http.Request) {
 	spaceID := r.PathValue("spaceID")
 	nameInput := strings.TrimSpace(r.FormValue("name"))
+	currencyInput := currency.Normalize(r.FormValue("currency"))
+	if currencyInput == "" {
+		currencyInput = currency.Default
+	}
 
 	formProps := forms.CreateAccountProps{
 		SpaceID:  spaceID,
 		Name:     nameInput,
+		Currency: currencyInput,
 	}
 
+	hasErr := false
 	if nameInput == "" {
 		formProps.NameErr = "Account name is required."
+		hasErr = true
+	}
+	if !currency.IsValid(currencyInput) {
+		formProps.CurrencyErr = "Choose a supported currency."
+		hasErr = true
+	}
+	if hasErr {
 		ui.Render(w, r, forms.CreateAccount(formProps))
 		return
 	}
@@ -285,7 +300,7 @@ func (h *spaceHandler) HandleCreateAccount(w http.ResponseWriter, r *http.Reques
 	if user != nil {
 		actorID = user.ID
 	}
-	account, err := h.accountService.CreateAccount(spaceID, nameInput, actorID)
+	account, err := h.accountService.CreateAccount(spaceID, nameInput, currencyInput, actorID)
 	if err != nil {
 		slog.Error("failed to create account", "error", err, "space_id", spaceID)
 		formProps.GeneralErr = "Something went wrong. Please try again."
@@ -343,6 +358,7 @@ func (h *spaceHandler) SpaceAccountPage(w http.ResponseWriter, r *http.Request)
 		AccountID:                accountID,
 		AccountName:              account.Name,
 		AccountBalance:           account.Balance,
+		AccountCurrency:          account.Currency,
 		RecentTransactions:       recent,
 		NonEditableTransactionIDs: h.nonEditableTransactionIDs(recent),
 		AllocationSummary:        allocSummary,
@@ -769,11 +785,17 @@ func (h *spaceHandler) SpaceAccountSettingsPage(w http.ResponseWriter, r *http.R
 		SpaceName:       space.Name,
 		AccountID:       accountID,
 		AccountName:     account.Name,
+		AccountCurrency: account.Currency,
 		UpdateForm: forms.UpdateAccountProps{
 			SpaceID:   spaceID,
 			AccountID: accountID,
 			Name:      account.Name,
 		},
+		CurrencyForm: forms.ChangeAccountCurrencyProps{
+			SpaceID:         spaceID,
+			AccountID:       accountID,
+			CurrentCurrency: account.Currency,
+		},
 	}))
 }
 
@@ -836,6 +858,80 @@ func (h *spaceHandler) HandleRenameAccount(w http.ResponseWriter, r *http.Reques
 	ui.Render(w, r, forms.UpdateAccount(formProps))
 }
 
+func (h *spaceHandler) HandleChangeAccountCurrency(w http.ResponseWriter, r *http.Request) {
+	spaceID := r.PathValue("spaceID")
+	accountID := r.PathValue("accountID")
+
+	account, err := h.accountService.GetAccount(accountID)
+	if err != nil || account.SpaceID != spaceID {
+		ui.RenderError(w, r, "Account not found", http.StatusNotFound)
+		return
+	}
+
+	newCurrencyInput := currency.Normalize(r.FormValue("new_currency"))
+	rateInput := strings.TrimSpace(r.FormValue("rate"))
+
+	formProps := forms.ChangeAccountCurrencyProps{
+		SpaceID:         spaceID,
+		AccountID:       accountID,
+		CurrentCurrency: account.Currency,
+		NewCurrency:     newCurrencyInput,
+		ConversionRate:  rateInput,
+	}
+
+	hasErr := false
+	if newCurrencyInput == "" {
+		formProps.NewCurrencyErr = "Choose a currency."
+		hasErr = true
+	} else if !currency.IsValid(newCurrencyInput) {
+		formProps.NewCurrencyErr = "Choose a supported currency."
+		hasErr = true
+	} else if newCurrencyInput == account.Currency {
+		formProps.NewCurrencyErr = "Choose a different currency."
+		hasErr = true
+	}
+
+	var rate decimal.Decimal
+	if rateInput == "" {
+		formProps.RateErr = "Conversion rate is required."
+		hasErr = true
+	} else {
+		r, err := decimal.NewFromString(rateInput)
+		if err != nil {
+			formProps.RateErr = "Enter a valid rate (e.g. 1.2345)."
+			hasErr = true
+		} else if !r.IsPositive() {
+			formProps.RateErr = "Rate must be greater than zero."
+			hasErr = true
+		} else {
+			rate = r
+		}
+	}
+
+	if hasErr {
+		ui.Render(w, r, forms.ChangeAccountCurrency(formProps))
+		return
+	}
+
+	user := ctxkeys.User(r.Context())
+	actorID := ""
+	if user != nil {
+		actorID = user.ID
+	}
+	if err := h.accountService.ChangeCurrency(accountID, newCurrencyInput, rate, actorID); err != nil {
+		slog.Error("failed to change account currency", "error", err, "account_id", accountID)
+		formProps.GeneralErr = "Something went wrong. Please try again."
+		ui.Render(w, r, forms.ChangeAccountCurrency(formProps))
+		return
+	}
+
+	formProps.CurrentCurrency = newCurrencyInput
+	formProps.NewCurrency = ""
+	formProps.ConversionRate = ""
+	formProps.SuccessMsg = "Currency updated. Balance and allocations were converted."
+	ui.Render(w, r, forms.ChangeAccountCurrency(formProps))
+}
+
 func (h *spaceHandler) HandleDeleteAccount(w http.ResponseWriter, r *http.Request) {
 	spaceID := r.PathValue("spaceID")
 	accountID := r.PathValue("accountID")
@@ -1550,6 +1646,7 @@ func (h *spaceHandler) SpaceCreateTransferPage(w http.ResponseWriter, r *http.Re
 		Form: forms.CreateTransferProps{
 			SpaceID:         spaceID,
 			SourceAccountID: accountID,
+			SourceCurrency:  account.Currency,
 			DestAccounts:    dests,
 			SourceAvailable: allocSummary.Available.StringFixedBank(2),
 			SourceAllocated: allocSummary.Allocated.StringFixedBank(2),
@@ -1579,16 +1676,19 @@ func (h *spaceHandler) HandleCreateTransfer(w http.ResponseWriter, r *http.Reque
 	titleInput := strings.TrimSpace(r.FormValue("title"))
 	amountInput := strings.TrimSpace(r.FormValue("amount"))
 	destInput := strings.TrimSpace(r.FormValue("destination"))
+	rateInput := strings.TrimSpace(r.FormValue("rate"))
 	dateInput := strings.TrimSpace(r.FormValue("date"))
 	descriptionInput := strings.TrimSpace(r.FormValue("description"))
 
 	formProps := forms.CreateTransferProps{
 		SpaceID:         spaceID,
 		SourceAccountID: accountID,
+		SourceCurrency:  source.Currency,
 		DestAccounts:    dests,
 		Title:           titleInput,
 		Amount:          amountInput,
 		DestAccountID:   destInput,
+		ConversionRate:  rateInput,
 		Date:            dateInput,
 		Description:     descriptionInput,
 	}
@@ -1627,6 +1727,7 @@ func (h *spaceHandler) HandleCreateTransfer(w http.ResponseWriter, r *http.Reque
 		}
 	}
 
+	var destCurrency string
 	if destInput == "" {
 		formProps.DestErr = "Choose a destination account."
 		hasErr = true
@@ -1639,6 +1740,27 @@ func (h *spaceHandler) HandleCreateTransfer(w http.ResponseWriter, r *http.Reque
 		if err != nil || destAcct.SpaceID != spaceID {
 			formProps.DestErr = "Destination account not found."
 			hasErr = true
+		} else {
+			destCurrency = destAcct.Currency
+		}
+	}
+
+	var rate decimal.Decimal
+	if destCurrency != "" && destCurrency != source.Currency {
+		if rateInput == "" {
+			formProps.RateErr = "Conversion rate is required for cross-currency transfers."
+			hasErr = true
+		} else {
+			r, err := decimal.NewFromString(rateInput)
+			if err != nil {
+				formProps.RateErr = "Enter a valid rate (e.g. 1.2345)."
+				hasErr = true
+			} else if !r.IsPositive() {
+				formProps.RateErr = "Rate must be greater than zero."
+				hasErr = true
+			} else {
+				rate = r
+			}
 		}
 	}
 
@@ -1671,6 +1793,7 @@ func (h *spaceHandler) HandleCreateTransfer(w http.ResponseWriter, r *http.Reque
 		DestAccountID:   destInput,
 		Title:           titleInput,
 		Amount:          amount,
+		ConversionRate:  rate,
 		OccurredAt:      occurredAt,
 		Description:     descriptionInput,
 		ActorID:         actorID,

internal/misc/currency/currency.go

@@ -0,0 +1,51 @@
+// Package currency provides ISO 4217 currency code validation and a small
+// curated list of supported codes for UI selection.
+package currency
+
+import "strings"
+
+const Default = "CAD"
+
+// supported is the curated list of ISO 4217 codes shown in the account creation
+// UI. Add codes here as users request them. Validation accepts any 3-letter
+// uppercase code in this list.
+var supported = []string{
+	"CAD",
+	"USD",
+	"EUR",
+	"GBP",
+	"JPY",
+	"AUD",
+	"CHF",
+	"CNY",
+	"HKD",
+	"MXN",
+	"NZD",
+	"SGD",
+	"INR",
+	"BRL",
+	"KRW",
+	"TWD",
+}
+
+func Supported() []string {
+	out := make([]string, len(supported))
+	copy(out, supported)
+	return out
+}
+
+// Normalize uppercases and trims the input. It does not validate.
+func Normalize(code string) string {
+	return strings.ToUpper(strings.TrimSpace(code))
+}
+
+// IsValid reports whether code is a supported ISO 4217 code (case-insensitive).
+func IsValid(code string) bool {
+	c := Normalize(code)
+	for _, s := range supported {
+		if s == c {
+			return true
+		}
+	}
+	return false
+}

internal/model/financial_management.go

@@ -11,6 +11,7 @@ type Account struct {
 	Name      string          `db:"name"`
 	SpaceID   string          `db:"space_id"`
 	Balance   decimal.Decimal `db:"balance"`
+	Currency  string          `db:"currency"`
 	CreatedAt time.Time       `db:"created_at"`
 	UpdatedAt time.Time       `db:"updated_at"`
 }

internal/model/space_audit_log.go

@@ -14,6 +14,7 @@ const (
 	SpaceAuditActionAccountCreated  SpaceAuditAction = "account.created"
 	SpaceAuditActionAccountRenamed  SpaceAuditAction = "account.renamed"
 	SpaceAuditActionAccountDeleted          SpaceAuditAction = "account.deleted"
+	SpaceAuditActionAccountCurrencyChanged  SpaceAuditAction = "account.currency_changed"
 	SpaceAuditActionAllocationCreated SpaceAuditAction = "allocation.created"
 	SpaceAuditActionAllocationUpdated SpaceAuditAction = "allocation.updated"
 	SpaceAuditActionAllocationDeleted SpaceAuditAction = "allocation.deleted"

internal/repository/account.go

@@ -3,11 +3,21 @@ package repository
 import (
 	"database/sql"
 	"errors"
+	"time"
 
 	"git.juancwu.dev/juancwu/budgit/internal/model"
 	"github.com/jmoiron/sqlx"
+	"github.com/shopspring/decimal"
 )
 
+// AllocationConversion describes the converted amount/target for a single
+// allocation row when an account's currency is changed.
+type AllocationConversion struct {
+	ID           string
+	Amount       decimal.Decimal
+	TargetAmount *decimal.Decimal
+}
+
 var ErrAccountNotFound = errors.New("account not found")
 
 type AccountRepository interface {
@@ -16,6 +26,9 @@ type AccountRepository interface {
 	BySpaceID(spaceID string) ([]*model.Account, error)
 	Rename(id, name string) error
 	Delete(id string) error
+	// ChangeCurrency atomically updates an account's currency and balance and
+	// rewrites each provided allocation's amount/target in the new currency.
+	ChangeCurrency(accountID, newCurrency string, newBalance decimal.Decimal, allocationConversions []AllocationConversion) error
 }
 
 type accountRepository struct {
@@ -27,9 +40,9 @@ func NewAccountRepository(db *sqlx.DB) AccountRepository {
 }
 
 func (r *accountRepository) Create(account *model.Account) error {
-	query := `INSERT INTO accounts (id, name, space_id, created_at, updated_at)
-	          VALUES ($1, $2, $3, $4, $5);`
-	_, err := r.db.Exec(query, account.ID, account.Name, account.SpaceID, account.CreatedAt, account.UpdatedAt)
+	query := `INSERT INTO accounts (id, name, space_id, currency, created_at, updated_at)
+	          VALUES ($1, $2, $3, $4, $5, $6);`
+	_, err := r.db.Exec(query, account.ID, account.Name, account.SpaceID, account.Currency, account.CreatedAt, account.UpdatedAt)
 	return err
 }
 
@@ -64,3 +77,24 @@ func (r *accountRepository) Delete(id string) error {
 	_, err := r.db.Exec(query, id)
 	return err
 }
+
+func (r *accountRepository) ChangeCurrency(accountID, newCurrency string, newBalance decimal.Decimal, allocationConversions []AllocationConversion) error {
+	return WithTx(r.db, func(tx *sqlx.Tx) error {
+		now := time.Now()
+		if _, err := tx.Exec(
+			`UPDATE accounts SET currency = $1, balance = $2, updated_at = $3 WHERE id = $4;`,
+			newCurrency, newBalance, now, accountID,
+		); err != nil {
+			return err
+		}
+		for _, c := range allocationConversions {
+			if _, err := tx.Exec(
+				`UPDATE allocations SET amount = $1, target_amount = $2, updated_at = $3 WHERE id = $4;`,
+				c.Amount, c.TargetAmount, now, c.ID,
+			); err != nil {
+				return err
+			}
+		}
+		return nil
+	})
+}

internal/routes/routes.go

@@ -117,6 +117,7 @@ func SetupRoutes(a *app.App) http.Handler {
 					g.Get("/transactions/{transactionID}/activity", spaceH.SpaceTransactionActivityPage).Name("page.app.spaces.space.accounts.account.transactions.transaction.activity")
 					g.Get("/settings", spaceH.SpaceAccountSettingsPage).Name("page.app.spaces.space.accounts.account.settings")
 					g.Post("/settings/rename", spaceH.HandleRenameAccount).Name("action.app.spaces.space.accounts.account.settings.rename")
+					g.Post("/settings/currency", spaceH.HandleChangeAccountCurrency).Name("action.app.spaces.space.accounts.account.settings.currency")
 					g.Post("/settings/delete", spaceH.HandleDeleteAccount).Name("action.app.spaces.space.accounts.account.settings.delete")
 					g.Get("/bills/create", spaceH.SpaceCreateBillPage).Name("page.app.spaces.space.accounts.account.bills.create")
 					g.Post("/bills/create", spaceH.HandleCreateBill).Name("action.app.spaces.space.accounts.account.bills.create")

internal/service/account.go

@@ -4,15 +4,18 @@ import (
 	"fmt"
 	"time"
 
+	"git.juancwu.dev/juancwu/budgit/internal/misc/currency"
 	"git.juancwu.dev/juancwu/budgit/internal/model"
 	"git.juancwu.dev/juancwu/budgit/internal/repository"
 	"github.com/google/uuid"
+	"github.com/shopspring/decimal"
 )
 
 const DefaultAccountName = "Money Account"
 
 type AccountService struct {
 	accountRepo    repository.AccountRepository
+	allocationRepo repository.AllocationRepository
 	auditSvc       *SpaceAuditLogService
 }
 
@@ -20,12 +23,18 @@ func NewAccountService(accountRepo repository.AccountRepository) *AccountService
 	return &AccountService{accountRepo: accountRepo}
 }
 
+// SetAllocationRepository wires the allocation repository after construction.
+// Required for currency conversion to rewrite allocation amounts atomically.
+func (s *AccountService) SetAllocationRepository(repo repository.AllocationRepository) {
+	s.allocationRepo = repo
+}
+
 // SetAuditLogger wires the audit log service after construction.
 func (s *AccountService) SetAuditLogger(audit *SpaceAuditLogService) {
 	s.auditSvc = audit
 }
 
-func (s *AccountService) CreateAccount(spaceID, name, actorID string) (*model.Account, error) {
+func (s *AccountService) CreateAccount(spaceID, name, currencyCode, actorID string) (*model.Account, error) {
 	if spaceID == "" {
 		return nil, fmt.Errorf("space id is required")
 	}
@@ -33,11 +42,20 @@ func (s *AccountService) CreateAccount(spaceID, name, actorID string) (*model.Ac
 		return nil, fmt.Errorf("account name cannot be empty")
 	}
 
+	code := currency.Normalize(currencyCode)
+	if code == "" {
+		code = currency.Default
+	}
+	if !currency.IsValid(code) {
+		return nil, fmt.Errorf("unsupported currency code: %s", currencyCode)
+	}
+
 	now := time.Now()
 	account := &model.Account{
 		ID:        uuid.NewString(),
 		Name:      name,
 		SpaceID:   spaceID,
+		Currency:  code,
 		CreatedAt: now,
 		UpdatedAt: now,
 	}
@@ -51,6 +69,7 @@ func (s *AccountService) CreateAccount(spaceID, name, actorID string) (*model.Ac
 		Metadata: map[string]any{
 			"account_id":   account.ID,
 			"account_name": account.Name,
+			"currency":     account.Currency,
 		},
 	})
 	return account, nil
@@ -118,6 +137,73 @@ func (s *AccountService) DeleteAccount(id, actorID string) error {
 	return nil
 }
 
+// ChangeCurrency converts the account's currency. Every value held in the old
+// currency (account balance, allocation amounts and targets) is multiplied by
+// rate and rounded to 2 decimals. The whole change is applied in a single SQL
+// transaction so the account never appears in a half-converted state.
+//
+// rate is "1 oldCurrency = rate newCurrency". Same-currency changes are
+// rejected; callers should treat rate as required and positive.
+func (s *AccountService) ChangeCurrency(accountID, newCurrencyCode string, rate decimal.Decimal, actorID string) error {
+	if accountID == "" {
+		return fmt.Errorf("account id is required")
+	}
+	code := currency.Normalize(newCurrencyCode)
+	if !currency.IsValid(code) {
+		return fmt.Errorf("unsupported currency code: %s", newCurrencyCode)
+	}
+	if !rate.IsPositive() {
+		return fmt.Errorf("conversion rate must be greater than zero")
+	}
+
+	account, err := s.accountRepo.ByID(accountID)
+	if err != nil {
+		return fmt.Errorf("failed to load account: %w", err)
+	}
+	if account.Currency == code {
+		return fmt.Errorf("account is already in %s", code)
+	}
+
+	allocations, err := s.allocationRepo.ByAccountID(accountID)
+	if err != nil {
+		return fmt.Errorf("failed to load allocations: %w", err)
+	}
+
+	newBalance := account.Balance.Mul(rate).Round(2)
+	conversions := make([]repository.AllocationConversion, 0, len(allocations))
+	for _, a := range allocations {
+		c := repository.AllocationConversion{
+			ID:     a.ID,
+			Amount: a.Amount.Mul(rate).Round(2),
+		}
+		if a.TargetAmount != nil {
+			t := a.TargetAmount.Mul(rate).Round(2)
+			c.TargetAmount = &t
+		}
+		conversions = append(conversions, c)
+	}
+
+	if err := s.accountRepo.ChangeCurrency(accountID, code, newBalance, conversions); err != nil {
+		return fmt.Errorf("failed to change currency: %w", err)
+	}
+
+	s.auditSvc.Record(RecordOptions{
+		SpaceID: account.SpaceID,
+		ActorID: actorID,
+		Action:  model.SpaceAuditActionAccountCurrencyChanged,
+		Metadata: map[string]any{
+			"account_id":      accountID,
+			"account_name":    account.Name,
+			"old_currency":    account.Currency,
+			"new_currency":    code,
+			"conversion_rate": rate.String(),
+			"old_balance":     account.Balance.StringFixedBank(2),
+			"new_balance":     newBalance.StringFixedBank(2),
+		},
+	})
+	return nil
+}
+
 func (s *AccountService) GetAccountsForSpace(spaceID string) ([]*model.Account, error) {
 	accounts, err := s.accountRepo.BySpaceID(spaceID)
 	if err != nil {

internal/service/account_test.go

@@ -22,7 +22,7 @@ func TestAccountService_CreateAccount_RecordsAudit(t *testing.T) {
 		user := testutil.CreateTestUser(t, dbi.DB, "acct-create-audit@example.com", nil)
 		space := testutil.CreateTestSpace(t, dbi.DB, user.ID, "S")
 
-		account, err := svc.CreateAccount(space.ID, "Checking", user.ID)
+		account, err := svc.CreateAccount(space.ID, "Checking", "CAD", user.ID)
 		require.NoError(t, err)
 
 		logs, err := auditRepo.ListAccountEvents(account.ID, 10, 0)
@@ -104,7 +104,7 @@ func TestAccountService_NoAuditLoggerSet_DoesNotPanic(t *testing.T) {
 		user := testutil.CreateTestUser(t, dbi.DB, "no-audit@example.com", nil)
 		space := testutil.CreateTestSpace(t, dbi.DB, user.ID, "S")
 
-		account, err := svc.CreateAccount(space.ID, "x", user.ID)
+		account, err := svc.CreateAccount(space.ID, "x", "", user.ID)
 		require.NoError(t, err)
 		require.NoError(t, svc.RenameAccount(account.ID, "y", user.ID))
 		require.NoError(t, svc.DeleteAccount(account.ID, user.ID))

internal/service/auth.go

@@ -363,7 +363,7 @@ func (s *AuthService) CompleteOnboarding(userID, name string) error {
 			return fmt.Errorf("failed to create onboarding space: %w", err)
 		}
 
-		if _, err := s.accountService.CreateAccount(space.ID, DefaultAccountName, userID); err != nil {
+		if _, err := s.accountService.CreateAccount(space.ID, DefaultAccountName, "", userID); err != nil {
 			if delErr := s.spaceService.DeleteSpace(space.ID, userID); delErr != nil {
 				slog.Error("failed to roll back space after account creation error",
 					"space_id", space.ID, "error", delErr)

internal/service/transaction.go

@@ -189,6 +189,10 @@ type TransferInput struct {
 	DestAccountID   string
 	Title           string
 	Amount          decimal.Decimal
+	// ConversionRate is the rate that converts one unit of the source currency
+	// into the destination currency. Required when source and destination
+	// accounts have different currencies; ignored otherwise. Must be positive.
+	ConversionRate decimal.Decimal
 	OccurredAt     time.Time
 	Description    string
 	ActorID        string
@@ -235,6 +239,18 @@ func (s *TransactionService) Transfer(input TransferInput) (*TransferResult, err
 		return nil, fmt.Errorf("failed to load destination account: %w", err)
 	}
 
+	// Cross-currency transfers require a conversion rate; same-currency
+	// transfers ignore it (or, for symmetry, accept rate=1).
+	destAmount := input.Amount
+	rate := decimal.NewFromInt(1)
+	if source.Currency != dest.Currency {
+		if !input.ConversionRate.IsPositive() {
+			return nil, fmt.Errorf("conversion rate is required when transferring between accounts of different currencies")
+		}
+		rate = input.ConversionRate
+		destAmount = input.Amount.Mul(rate).Round(2)
+	}
+
 	now := time.Now()
 	var description *string
 	if d := strings.TrimSpace(input.Description); d != "" {
@@ -254,7 +270,7 @@ func (s *TransactionService) Transfer(input TransferInput) (*TransferResult, err
 	}
 	deposit := &model.Transaction{
 		ID:          uuid.NewString(),
-		Value:       input.Amount,
+		Value:       destAmount,
 		Type:        model.TransactionTypeDeposit,
 		AccountID:   dest.ID,
 		Title:       title,
@@ -265,7 +281,7 @@ func (s *TransactionService) Transfer(input TransferInput) (*TransferResult, err
 	}
 
 	sourceNewBalance := source.Balance.Sub(input.Amount)
-	destNewBalance := dest.Balance.Add(input.Amount)
+	destNewBalance := dest.Balance.Add(destAmount)
 
 	if err := s.transactionRepo.TransferAtomic(withdrawal, deposit, sourceNewBalance, destNewBalance); err != nil {
 		return nil, fmt.Errorf("failed to record transfer: %w", err)
@@ -287,6 +303,10 @@ func (s *TransactionService) Transfer(input TransferInput) (*TransferResult, err
 			"transfer_pair_id":    deposit.ID,
 			"transfer_other_acct": deposit.AccountID,
 			"transfer_other_name": dest.Name,
+			"source_currency":     source.Currency,
+			"dest_currency":       dest.Currency,
+			"conversion_rate":     rate.String(),
+			"dest_amount":         destAmount.StringFixedBank(2),
 		},
 	})
 	s.auditSvc.Record(TransactionRecordOptions{
@@ -302,6 +322,10 @@ func (s *TransactionService) Transfer(input TransferInput) (*TransferResult, err
 			"transfer_pair_id":    withdrawal.ID,
 			"transfer_other_acct": withdrawal.AccountID,
 			"transfer_other_name": source.Name,
+			"source_currency":     source.Currency,
+			"dest_currency":       dest.Currency,
+			"conversion_rate":     rate.String(),
+			"source_amount":       input.Amount.StringFixedBank(2),
 		},
 	})
 

internal/testutil/seed.go

@@ -89,12 +89,13 @@ func CreateTestAccount(t *testing.T, db *sqlx.DB, spaceID, name string) *model.A
 		Name:      name,
 		SpaceID:   spaceID,
 		Balance:   decimal.Zero,
+		Currency:  "CAD",
 		CreatedAt: now,
 		UpdatedAt: now,
 	}
 	_, err := db.Exec(
-		`INSERT INTO accounts (id, name, space_id, balance, created_at, updated_at) VALUES ($1, $2, $3, $4, $5, $6)`,
-		account.ID, account.Name, account.SpaceID, account.Balance, account.CreatedAt, account.UpdatedAt,
+		`INSERT INTO accounts (id, name, space_id, balance, currency, created_at, updated_at) VALUES ($1, $2, $3, $4, $5, $6, $7)`,
+		account.ID, account.Name, account.SpaceID, account.Balance, account.Currency, account.CreatedAt, account.UpdatedAt,
 	)
 	if err != nil {
 		t.Fatalf("CreateTestAccount: %v", err)

internal/ui/blocks/account_card.templ

@@ -10,6 +10,7 @@ type AccountCardInfo struct {
 	ID       string
 	Name     string
 	Balance  decimal.Decimal
+	Currency string
 }
 
 templ AccountCard(info AccountCardInfo) {
@@ -22,7 +23,7 @@ templ AccountCard(info AccountCardInfo) {
 				<div>
 					<p class="font-semibold">{ info.Name }</p>
 					<p class="text-xs text-muted-foreground">
-						${ info.Balance.StringFixedBank(2) }
+						${ info.Balance.StringFixedBank(2) } { info.Currency }
 					</p>
 				</div>
 			</div>

internal/ui/forms/change_account_currency.templ

@@ -0,0 +1,117 @@
+package forms
+
+import "git.juancwu.dev/juancwu/budgit/internal/misc/currency"
+import "git.juancwu.dev/juancwu/budgit/internal/routeurl"
+import "git.juancwu.dev/juancwu/budgit/internal/ui/components/button"
+import "git.juancwu.dev/juancwu/budgit/internal/ui/components/card"
+import "git.juancwu.dev/juancwu/budgit/internal/ui/components/form"
+import "git.juancwu.dev/juancwu/budgit/internal/ui/components/input"
+
+type ChangeAccountCurrencyProps struct {
+	SpaceID         string
+	AccountID       string
+	CurrentCurrency string
+
+	NewCurrency    string
+	ConversionRate string
+
+	NewCurrencyErr string
+	RateErr        string
+	GeneralErr     string
+	SuccessMsg     string
+}
+
+templ ChangeAccountCurrency(props ChangeAccountCurrencyProps) {
+	<form
+		id="change-currency-form"
+		hx-post={ routeurl.URL("action.app.spaces.space.accounts.account.settings.currency", "spaceID", props.SpaceID, "accountID", props.AccountID) }
+		hx-swap="outerHTML"
+	>
+		@card.Card(card.Props{Class: "rounded-sm"}) {
+			@card.Header() {
+				@card.Title() {
+					Currency
+				}
+				@card.Description() {
+					Currently { props.CurrentCurrency }. Changing the currency converts the balance and every allocation at the rate you provide.
+				}
+			}
+			@card.Content(card.ContentProps{Class: "space-y-4"}) {
+				if props.GeneralErr != "" {
+					@form.Message(form.MessageProps{Variant: form.MessageVariantError}) {
+						{ props.GeneralErr }
+					}
+				}
+				if props.SuccessMsg != "" {
+					@form.Message(form.MessageProps{Variant: form.MessageVariantInfo}) {
+						{ props.SuccessMsg }
+					}
+				}
+				@form.Item() {
+					@form.Label(form.LabelProps{For: "new_currency"}) {
+						New currency
+					}
+					{{ selected := props.NewCurrency }}
+					<select
+						id="new_currency"
+						name="new_currency"
+						class={ "flex h-9 w-full items-center rounded-sm border bg-transparent px-3 py-1 text-sm shadow-sm focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring",
+							templ.KV("border-destructive", props.NewCurrencyErr != ""),
+							templ.KV("border-input", props.NewCurrencyErr == "") }
+						required
+					>
+						<option value="" selected?={ selected == "" }>Select a currency…</option>
+						for _, code := range currency.Supported() {
+							if code != props.CurrentCurrency {
+								<option value={ code } selected?={ selected == code }>{ code }</option>
+							}
+						}
+					</select>
+					if props.NewCurrencyErr != "" {
+						@form.Message(form.MessageProps{Variant: form.MessageVariantError}) {
+							{ props.NewCurrencyErr }
+						}
+					}
+				}
+				@form.Item() {
+					@form.Label(form.LabelProps{For: "rate"}) {
+						Conversion rate
+					}
+					<div class="flex items-center gap-2">
+						<span class="text-sm text-muted-foreground">1 { props.CurrentCurrency } =</span>
+						@input.Input(input.Props{
+							ID:          "rate",
+							Name:        "rate",
+							Type:        input.TypeNumber,
+							Placeholder: "1.000000",
+							Class:       "rounded-sm",
+							Value:       props.ConversionRate,
+							HasError:    props.RateErr != "",
+							Required:    true,
+							Attributes: templ.Attributes{
+								"step":         "0.000001",
+								"min":          "0",
+								"inputmode":    "decimal",
+								"autocomplete": "off",
+							},
+						})
+						<span class="text-sm text-muted-foreground">new currency</span>
+					</div>
+					if props.RateErr != "" {
+						@form.Message(form.MessageProps{Variant: form.MessageVariantError}) {
+							{ props.RateErr }
+						}
+					}
+					@form.Description() {
+						Balance and allocation amounts are multiplied by this rate and rounded to 2 decimals.
+					}
+				}
+			}
+			@card.Footer(card.FooterProps{Class: "flex justify-end gap-2"}) {
+				@button.Button(button.Props{Type: button.TypeSubmit}) {
+					Change currency
+				}
+			}
+		}
+	</form>
+}

internal/ui/forms/create_account.templ

@@ -1,5 +1,6 @@
 package forms
 
+import "git.juancwu.dev/juancwu/budgit/internal/misc/currency"
 import "git.juancwu.dev/juancwu/budgit/internal/routeurl"
 import "git.juancwu.dev/juancwu/budgit/internal/ui/components/button"
 import "git.juancwu.dev/juancwu/budgit/internal/ui/components/card"
@@ -10,8 +11,10 @@ type CreateAccountProps struct {
 	SpaceID string
 
 	Name     string
+	Currency string
 
 	NameErr     string
+	CurrencyErr string
 	GeneralErr  string
 }
 
@@ -48,7 +51,35 @@ templ CreateAccount(props CreateAccountProps) {
 						}
 					}
 					@form.Description() {
-						You can rename the account later. Starts with a $0.00 balance.
+						You can rename the account later. Starts with a 0.00 balance.
+					}
+				}
+				{{
+					selected := props.Currency
+					if selected == "" {
+						selected = currency.Default
+					}
+				}}
+				@form.Item() {
+					@form.Label(form.LabelProps{For: "currency"}) {
+						Currency
+					}
+					<select
+						id="currency"
+						name="currency"
+						class={ "flex h-9 w-full items-center rounded-sm border bg-transparent px-3 py-1 text-sm shadow-sm focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring",
+							templ.KV("border-destructive", props.CurrencyErr != ""),
+							templ.KV("border-input", props.CurrencyErr == "") }
+						required
+					>
+						for _, code := range currency.Supported() {
+							<option value={ code } selected?={ selected == code }>{ code }</option>
+						}
+					</select>
+					if props.CurrencyErr != "" {
+						@form.Message(form.MessageProps{Variant: form.MessageVariantError}) {
+							{ props.CurrencyErr }
+						}
 					}
 				}
 			}

internal/ui/forms/create_transfer.templ

@@ -12,6 +12,7 @@ import "git.juancwu.dev/juancwu/budgit/internal/ui/utils"
 type CreateTransferProps struct {
 	SpaceID         string
 	SourceAccountID string
+	SourceCurrency  string
 
 	// DestAccounts is the list of other accounts in the same space the user
 	// can transfer to. Excludes the source account.
@@ -26,12 +27,14 @@ type CreateTransferProps struct {
 	Title          string
 	Amount         string
 	DestAccountID  string
+	ConversionRate string
 	Date           string
 	Description    string
 
 	TitleErr   string
 	AmountErr  string
 	DestErr    string
+	RateErr    string
 	DateErr    string
 	GeneralErr string
 }
@@ -104,10 +107,26 @@ templ CreateTransfer(props CreateTransferProps) {
 								templ.KV("border-destructive", props.DestErr != ""),
 								templ.KV("border-input", props.DestErr == "") }
 							required
+							data-source-currency={ props.SourceCurrency }
+							_="on change
+								set opt to my.options[my.selectedIndex]
+								set destCur to opt.getAttribute('data-currency') or ''
+								set srcCur to @data-source-currency of me
+								if destCur is not '' and destCur is not srcCur
+									remove .hidden from #rate-row
+									set #rate-dest-currency.innerText to destCur
+									set #rate-source-currency.innerText to srcCur
+								else
+									add .hidden to #rate-row
+								end"
 						>
 							<option value="" selected?={ props.DestAccountID == "" }>Select an account…</option>
 							for _, a := range props.DestAccounts {
-								<option value={ a.ID } selected?={ props.DestAccountID == a.ID }>{ a.Name }</option>
+								<option
+									value={ a.ID }
+									data-currency={ a.Currency }
+									selected?={ props.DestAccountID == a.ID }
+								>{ a.Name } ({ a.Currency })</option>
 							}
 						</select>
 					}
@@ -167,6 +186,57 @@ templ CreateTransfer(props CreateTransferProps) {
 						}
 					}
 				</div>
+				{{
+					selectedDestCurrency := ""
+					for _, a := range props.DestAccounts {
+						if a.ID == props.DestAccountID {
+							selectedDestCurrency = a.Currency
+							break
+						}
+					}
+					rateRowHidden := selectedDestCurrency == "" || selectedDestCurrency == props.SourceCurrency
+					rateRowClasses := []string{"space-y-2 rounded-md border p-4 bg-muted/30"}
+					if rateRowHidden {
+						rateRowClasses = append(rateRowClasses, "hidden")
+					}
+				}}
+				<div id="rate-row" class={ utils.TwMerge(rateRowClasses...) }>
+					<p class="text-sm">
+						Source and destination use different currencies. Set the conversion rate.
+					</p>
+					@form.Item() {
+						@form.Label(form.LabelProps{For: "rate"}) {
+							Conversion rate
+						}
+						<div class="flex items-center gap-2">
+							<span class="text-sm text-muted-foreground">1 <span id="rate-source-currency">{ props.SourceCurrency }</span> =</span>
+							@input.Input(input.Props{
+								ID:          "rate",
+								Name:        "rate",
+								Type:        input.TypeNumber,
+								Placeholder: "1.00",
+								Class:       "rounded-sm",
+								Value:       props.ConversionRate,
+								HasError:    props.RateErr != "",
+								Attributes: templ.Attributes{
+									"step":         "0.000001",
+									"min":          "0",
+									"inputmode":    "decimal",
+									"autocomplete": "off",
+								},
+							})
+							<span class="text-sm text-muted-foreground" id="rate-dest-currency">{ selectedDestCurrency }</span>
+						</div>
+						if props.RateErr != "" {
+							@form.Message(form.MessageProps{Variant: form.MessageVariantError}) {
+								{ props.RateErr }
+							}
+						}
+						@form.Description() {
+							The destination account will be credited the converted amount, rounded to 2 decimals.
+						}
+					}
+				</div>
 				@form.Item() {
 					@form.Label(form.LabelProps{For: "description"}) {
 						Description

internal/ui/pages/space_account.templ

@@ -6,6 +6,7 @@ import "git.juancwu.dev/juancwu/budgit/internal/routeurl"
 import "git.juancwu.dev/juancwu/budgit/internal/service"
 import "git.juancwu.dev/juancwu/budgit/internal/ui/blocks"
 import "git.juancwu.dev/juancwu/budgit/internal/ui/layouts"
+import "git.juancwu.dev/juancwu/budgit/internal/ui/components/badge"
 import "git.juancwu.dev/juancwu/budgit/internal/ui/components/card"
 import "git.juancwu.dev/juancwu/budgit/internal/ui/components/button"
 import "git.juancwu.dev/juancwu/budgit/internal/ui/components/icon"
@@ -17,6 +18,7 @@ type SpaceAccountPageProps struct {
 	AccountID                 string
 	AccountName               string
 	AccountBalance            decimal.Decimal
+	AccountCurrency           string
 	RecentTransactions        []*model.Transaction
 	NonEditableTransactionIDs map[string]bool
 	AllocationSummary         *service.AllocationSummary
@@ -39,12 +41,20 @@ templ SpaceAccountPage(props SpaceAccountPageProps) {
 				}) {
 					@card.Header() {
 						@card.Title() {
-							{ props.AccountName }
+							<div class="flex items-center gap-3">
+								<span>{ props.AccountName }</span>
+								@badge.Badge(badge.Props{Variant: badge.VariantSecondary, Class: "text-xs font-medium"}) {
+									{ props.AccountCurrency }
+								}
+							</div>
 						}
 					}
 					@card.Content() {
-						<h1 class={ utils.TwMerge(balanceTextClasses...) }>${ utils.FormatDecimalWithThousands(props.AccountBalance.StringFixedBank(2)) }</h1>
-						<p class="text-sm text-muted-foreground">Account Balance</p>
+						<h1 class={ utils.TwMerge(balanceTextClasses...) }>
+							${ utils.FormatDecimalWithThousands(props.AccountBalance.StringFixedBank(2)) }
+							<span class="text-xl font-semibold text-muted-foreground ml-2">{ props.AccountCurrency }</span>
+						</h1>
+						<p class="text-sm text-muted-foreground">Account Balance ({ props.AccountCurrency })</p>
 					}
 				}
 				@card.Card(card.Props{Class: "rounded-sm col-span-full md:col-span-4"}) {

internal/ui/pages/space_account_settings.templ

@@ -13,7 +13,9 @@ type SpaceAccountSettingsPageProps struct {
 	SpaceName       string
 	AccountID       string
 	AccountName     string
+	AccountCurrency string
 	UpdateForm      forms.UpdateAccountProps
+	CurrencyForm    forms.ChangeAccountCurrencyProps
 }
 
 templ SpaceAccountSettingsPage(props SpaceAccountSettingsPageProps) {
@@ -32,6 +34,7 @@ templ SpaceAccountSettingsPage(props SpaceAccountSettingsPageProps) {
 				</p>
 			</div>
 			@forms.UpdateAccount(props.UpdateForm)
+			@forms.ChangeAccountCurrency(props.CurrencyForm)
 			@card.Card(card.Props{Class: "rounded-sm border-destructive"}) {
 				@card.Header() {
 					@card.Title(card.TitleProps{Class: "text-destructive"}) {