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Swift 3
currency formatting
double to currency
Swift programming
iOS development

How to format a Double into Currency - Swift 3

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Introduction

To format a Double as currency in Swift, use NumberFormatter with numberStyle = .currency. It automatically applies the correct currency symbol, decimal separator, grouping separator, and decimal places based on the user's locale. For Swift 5.5+ (iOS 15+), the formatted(.currency(code:)) API is even simpler. Avoid manual string interpolation like "$\(String(format: "%.2f", amount))" — it ignores locale, breaks for non-USD currencies, and uses the wrong separators for many regions.

NumberFormatter (All Swift Versions)

swift
1let amount = 1234.56
2
3let formatter = NumberFormatter()
4formatter.numberStyle = .currency
5
6let formatted = formatter.string(from: NSNumber(value: amount))
7// US locale: "$1,234.56"
8// German locale: "1.234,56 €"
9// Japanese locale: "¥1,235"

NumberFormatter uses the device's current locale by default. It handles currency symbols, grouping separators, decimal separators, and the number of fraction digits automatically.

Specifying a Locale

swift
1let amount = 1234.56
2
3// US Dollar
4let usFormatter = NumberFormatter()
5usFormatter.numberStyle = .currency
6usFormatter.locale = Locale(identifier: "en_US")
7print(usFormatter.string(from: NSNumber(value: amount))!)
8// "$1,234.56"
9
10// Euro (Germany)
11let deFormatter = NumberFormatter()
12deFormatter.numberStyle = .currency
13deFormatter.locale = Locale(identifier: "de_DE")
14print(deFormatter.string(from: NSNumber(value: amount))!)
15// "1.234,56 €"
16
17// Yen (Japan)
18let jpFormatter = NumberFormatter()
19jpFormatter.numberStyle = .currency
20jpFormatter.locale = Locale(identifier: "ja_JP")
21print(jpFormatter.string(from: NSNumber(value: amount))!)
22// "¥1,235" (no decimal places for yen)
23
24// Pound (UK)
25let ukFormatter = NumberFormatter()
26ukFormatter.numberStyle = .currency
27ukFormatter.locale = Locale(identifier: "en_GB")
28print(ukFormatter.string(from: NSNumber(value: amount))!)
29// "£1,234.56"

Specifying a Currency Code

To show a specific currency regardless of locale:

swift
1let formatter = NumberFormatter()
2formatter.numberStyle = .currency
3formatter.currencyCode = "EUR"  // ISO 4217 currency code
4formatter.locale = Locale(identifier: "en_US")
5
6print(formatter.string(from: NSNumber(value: 1234.56))!)
7// "€1,234.56" (Euro symbol with US formatting)

Formatted API (iOS 15+ / Swift 5.5+)

swift
1let amount = 1234.56
2
3// Uses device locale
4let formatted = amount.formatted(.currency(code: "USD"))
5// "$1,234.56"
6
7// Specific locale
8let euroFormatted = amount.formatted(
9    .currency(code: "EUR")
10    .locale(Locale(identifier: "de_DE"))
11)
12// "1.234,56 €"
13
14// No fraction digits
15let yenFormatted = amount.formatted(
16    .currency(code: "JPY")
17    .precision(.fractionLength(0))
18)
19// "¥1,235"

Reusable Extension

swift
1extension Double {
2    func toCurrency(locale: Locale = .current) -> String {
3        let formatter = NumberFormatter()
4        formatter.numberStyle = .currency
5        formatter.locale = locale
6        return formatter.string(from: NSNumber(value: self)) ?? "$0.00"
7    }
8
9    func toCurrency(code: String, locale: Locale = .current) -> String {
10        let formatter = NumberFormatter()
11        formatter.numberStyle = .currency
12        formatter.currencyCode = code
13        formatter.locale = locale
14        return formatter.string(from: NSNumber(value: self)) ?? "$0.00"
15    }
16}
17
18// Usage
19let price = 99.99
20print(price.toCurrency())                                    // "$99.99" (US locale)
21print(price.toCurrency(code: "GBP"))                        // "£99.99"
22print(price.toCurrency(locale: Locale(identifier: "fr_FR"))) // "99,99 €"

Customizing Decimal Places

swift
1let formatter = NumberFormatter()
2formatter.numberStyle = .currency
3formatter.minimumFractionDigits = 0
4formatter.maximumFractionDigits = 0
5
6print(formatter.string(from: NSNumber(value: 1234.56))!)
7// "$1,235" (rounded, no decimals)
8
9// Force 3 decimal places
10formatter.minimumFractionDigits = 3
11formatter.maximumFractionDigits = 3
12print(formatter.string(from: NSNumber(value: 1234.5))!)
13// "$1,234.500"

Using in SwiftUI

swift
1import SwiftUI
2
3struct PriceView: View {
4    let amount: Double
5
6    var body: some View {
7        Text(amount, format: .currency(code: "USD"))
8            .font(.title)
9    }
10}
11
12// With a TextField for currency input
13struct PriceInputView: View {
14    @State private var price: Double = 0
15
16    var body: some View {
17        TextField("Price", value: $price,
18                  format: .currency(code: "USD"))
19            .keyboardType(.decimalPad)
20    }
21}

Common Pitfalls

  • Using string interpolation instead of NumberFormatter: "$\(String(format: "%.2f", amount))" hardcodes the dollar sign and uses . as the decimal separator. This is incorrect for most non-US locales where the comma is the decimal separator and the currency symbol differs or appears after the number.
  • Creating a new NumberFormatter on every call: NumberFormatter is expensive to initialize. In table views or collection views, create the formatter once and reuse it. Store it as a property or use a static instance.
  • Forgetting that Double has floating-point precision issues: 0.1 + 0.2 is not exactly 0.3 in floating point. For financial calculations, use Decimal or NSDecimalNumber instead of Double to avoid rounding errors that appear in formatted output.
  • Hardcoding a locale instead of using .current: Hardcoding Locale(identifier: "en_US") forces US formatting for all users. Use Locale.current unless you specifically need a fixed locale (like displaying prices in a specific currency for all users).
  • Not handling the optional return from formatter.string(from:): NumberFormatter.string(from:) returns an optional String?. If the value is NaN or infinity, it returns nil. Always provide a fallback with ?? "$0.00" or use a guard statement.

Summary

  • Use NumberFormatter with .currency style for locale-aware currency formatting
  • Use .formatted(.currency(code: "USD")) on iOS 15+ for a concise one-liner
  • Set currencyCode to format a specific currency regardless of locale
  • Create a reusable Double extension to avoid duplicating formatter setup
  • Use Decimal instead of Double for financial calculations to avoid floating-point rounding errors
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