FractionFormatter

A subclass of Apple's NumberFormatter that outputs pretty-printed Unicode fractions rather than decimals.

Requirements

• Swift 5+
• iOS 8+
• macOS 10.13+
• tvOS 9+
• watchOS 5+

Installation

Xcode

In Xcode, use File > Add Package Dependencies... and enter:

https://gitlab.com/davidwkeith/fractionformatter.git

Swift Package Manager

Add FractionFormatter to your Package.swift dependencies:

.package(url: "https://gitlab.com/davidwkeith/fractionformatter.git", from: "1.0.1")

Then add it to your target dependencies:

.target(
    name: "YourTarget",
    dependencies: ["FractionFormatter"]
)

Usage

FractionFormatter is a direct replacement for NumberFormatter:

let fractionFormatter = FractionFormatter()

fractionFormatter.string(from: NSNumber(value: 0.5))   // "½"
fractionFormatter.string(from: NSNumber(value: 0.123)) // "¹²³⁄₁₀₀₀"

It can also parse and normalize fraction-like strings:

fractionFormatter.double(from: "1½")                  // 1.5
fractionFormatter.double(from: "1 1/2")               // 1.5
fractionFormatter.string(from: "1 1/2")               // "1½"
fractionFormatter.string(from: "1½", as: .builtUp)   // "1 1/2"
fractionFormatter.string(from: "1½", as: .caseFraction) // "1 1/2" (case-fraction source text)

New Configuration Features

FractionFormatter now supports configurable parsing, formatting, and extensibility:

• Locale-aware parsing (parsingLocale, allowsLocaleAwareParsing)
• Precision/reduction policies (reductionPolicy)
• Negative formatting styles (negativeFormatStyle, negativeSignSymbol)
• Typography controls (unicodeFormattingStyle, separator properties)
• Custom vulgar-fraction glyph maps (vulgarFractionGlyphs)
• Measurement formatting helper with optional singular-unit workaround

let formatter = FractionFormatter()

// 1) Locale-aware parsing
formatter.parsingLocale = Locale(identifier: "fr_FR")
formatter.double(from: "1,5") // 1.5

// 2) Rational reduction control
formatter.reductionPolicy = .maxDenominator(16)
formatter.string(from: NSNumber(value: 2.2), as: .builtUp) // "2 1/5"

// 3) Negative display style
formatter.negativeFormatStyle = .parenthesized
formatter.string(from: NSNumber(value: -1.5)) // "(1½)"

// 4) Typography customization
formatter.unicodeFormattingStyle = .inline
formatter.unicodeWholeFractionSeparator = " "
formatter.unicodeDivisionSeparator = "/"
formatter.string(from: NSNumber(value: 1.123)) // "1 123/1000"

// 5) Custom glyphs
formatter.vulgarFractionGlyphs = [0.5: "⯪"]
formatter.string(from: NSNumber(value: 0.5)) // "⯪"
formatter.double(from: "⯪") // 0.5

Fraction Output Styles

FractionFormatter.FractionType supports:

• .unicode for Unicode output (for example, "1½" or "¹²³⁄₁₀₀₀")
• .builtUp for slash-separated output (for example, "1 1/2")
• .caseFraction for slash-separated source text intended for typographic case-fraction rendering (for example, "1 1/2")

On Apple platforms, you can request OpenType fraction features using attributed output:

let attributed = fractionFormatter.attributedString(
    from: NSNumber(value: 1.5),
    as: .caseFraction
)

Filing Feature Requests and Issues

The source is hosted on GitLab and mirrored on GitHub. If you find an issue or have a feature request, file it here.

Project Quality

• Changelog: CHANGELOG.md
• Release process: RELEASE_CHECKLIST.md
• Contributor guide: CONTRIBUTING.md
• Swift formatting config: .swift-format
• CI:
  • GitLab CI (.gitlab-ci.yml) runs lint/test/docs checks
  • GitHub Actions (.github/workflows/ci.yml) runs Linux + macOS test matrix

Known Issues

Radar FB7644708 - Pluralization and Number Formatting

When combined with Apple's MeasurementFormatter, there are issues with pluralization. For example, using NumberFormatter to format fractional feet outputs "0.5 feet" ("zero point five feet"). Replacing it with FractionFormatter can produce "½ feet", which is not the standard English form. Normally we say "half a foot" (or "one half of a foot"), so singular wording is preferred.

As of February 19, 2026 there is no public API on MeasurementFormatter to force singular unit words for fractional values while keeping full localized unit inflection.

Workarounds

1. Prefer .short/.medium unit styles (ft, kg, etc.) where singular/plural words are not shown.
2. For values where abs(value) < 1, use a custom localized phrase ("half a foot", "one half of a foot", etc.) from your app localization resources.
3. If you only need an English-style output path, post-process the unit word for that range.

You can also use the built-in helper:

let fractionFormatter = FractionFormatter()
let measurementFormatter = MeasurementFormatter()
measurementFormatter.unitStyle = .long
measurementFormatter.unitOptions = [.providedUnit]

let rendered = fractionFormatter.string(
    from: Measurement(value: 0.5, unit: UnitLength.feet),
    with: measurementFormatter,
    preferSingularUnitForProperFractions: true
)

Example (option 3):

import Foundation

func formatLength(_ value: Double, formatter: MeasurementFormatter) -> String {
    let measurement = Measurement(value: value, unit: UnitLength.feet)
    let formatted = formatter.string(from: measurement)

    guard abs(value) < 1 else { return formatted }

    // English-specific fallback for FB7644708.
    return formatted.replacingOccurrences(of: " feet", with: " foot")
}