How to convert units using Unit and Measurement

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

iOS 10 introduced a new system for calculating distance, length, area, volume, duration, and many more measurements. Let’s start with something simple. If you’re six feet tall, you’d create a Measurement instance like this:

let heightFeet = Measurement(value: 6, unit: UnitLength.feet)

Note that Swift can’t infer .feet to mean UnitLength.feet because there are lots of Unit subclasses as you’ll see soon.

Once you have a measurement ready, you can convert it to other units like this:

let heightInches = heightFeet.converted(to: UnitLength.inches)
let heightSensible = heightFeet.converted(to: UnitLength.meters)

You should see “72.0 in” and “1.8288 m” in your output, showing that the conversion process has worked.

The UnitLength class, like all unit subclasses, spans a huge range of units from old to futuristic. For example, you can convert feet to astronomical units, which is equal to the average distance between the Earth and the Sun, or about 150 million kilometers:

let heightAUs = heightFeet.converted(to: UnitLength.astronomicalUnits)

Once you’ve used one unit, the rest work identically. Here are some more examples to get you started:

// convert degrees to radians
let degrees = Measurement(value: 180, unit: UnitAngle.degrees)
let radians = degrees.converted(to: .radians)

// convert square meters to square centimeters
let squareMeters = Measurement(value: 4, unit: UnitArea.squareMeters)
let squareCentimeters = squareMeters.converted(to: .squareCentimeters)

// convert bushels to imperial teaspoons
let bushels = Measurement(value: 6, unit: UnitVolume.bushels)
let teaspoons = bushels.converted(to: .imperialTeaspoons)

Honestly, I have no idea what the bushels to imperial teaspoons ratio is, but it’s nice to be given the option!