How to display percentage values in SwiftUI

iOSdev / SwiftUI

How to display percentage values in SwiftUI

Lately, one user of my mood tracking app Emo requested that I add percentage values to my Statistics screen. Since displaying percentage values is a little harder than displaying numbers, I thought I would share a few ways of doing this in this article.

Let's say we have two Doubles and a total value equal to the sum of them.

let value1: Double = 1
let value2: Double = 2
let total = value1 + value2 // 3

Option 1

Now, if we want to check what percentage of the total is value1 and display this percentage in SwiftUI with 0 fraction digits, we could do something like this:

Text("\(String(format: "%.0f", (value1 / total * 100)))%")

Option 2

Since iOS 15, there is another way, though, and you may find it more readable:

Text((value1 / total).formatted(

Both options will result in the same text:
33% (for value1)
67% (for value2)

As you can probably guess, to change the precision to display 2 fraction digits in your percentages, you can modify both versions of the code as follows:

Text("\(String(format: "%.2f", (value1 / total * 100)))%") // %.2f

Text((value1 / total).formatted(
    .percent.precision(.fractionLength(2))) // fractionLength(2)

When you display fraction digits, you will notice another difference between both methods. The first one will always display percentage values the same way, as it's a hard-coded format (for example, 33.33%).

The second one, using the formatted() method, takes the user's locale into consideration. So the percentage value may be displayed as 33.33% or 33,33%, depending on the locale. The same applies to other locale-specific rules, like grouping separators (e.g., "10,000" in one country and "10 000" in another).

This means that the second option is not only more readable but also a better choice if the app is localized, as with the formatted() method, all the values will be properly formatted based on the user's locale.

UPDATE: Option 3

After I published this article, Łukasz Rutkowski reminded me on Mastodon about another similar method (which also formats numbers properly based on locale):

Text((value1 / total),
    format: .percent.precision(.fractionLength(0))

If you know another way to display percentages in SwiftUI, feel free to share it in the comments.

Thank you for reading!

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Thank you to Arnaud Joubay and Avery Vine for your input on app localization, and to Łukasz Rutkowski for suggesting Option 3.