Suppose you have a UILabel on your storyboard and you have created an IBOutlet for it in ViewController.swift / ViewController.m and named it labelOne.

To make the changes easily visible, change the backgroundColor and textColor of labelOne in the viewDidLoad method:

The function sizeToFit is used when you want to automatically resize a label based on the content stored within it.

Swift

labelOne.backgroundColor = UIColor.blueColor()
labelOne.textColor = UIColor.whiteColor()
labelOne.text = "Hello, World!"
labelOne.sizeToFit()

Swift 3

labelOne.backgroundColor = UIColor.blue
labelOne.textColor = UIColor.white
labelOne.text = "Hello, World!"
labelOne.sizeToFit()

Objective-C

labelOne.backgroundColor = [UIColor blueColor];
labelOne.textColor = [UIColor whiteColor];
labelOne.text = @"Hello, World!";
[labelOne sizeToFit];

The output for the above code is:

As you can see, there is no change as the text is perfectly fitting in labelOne. sizeToFit only changes the label’s frame.

Let’s change the text to a slightly longer one:

labelOne.text = "Hello, World! I’m glad to be alive!"

Now, labelOne looks like this:

Even calling sizeToFit doesn’t change anything. This is because by default, the numberOfLines shown by the UILabel is set to 1. Let’s change it to zero on the storyboard:

This time, when we run the app, labelOne appears correctly: