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UILabel
iOS
Swift
text formatting
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Setting UILabel text to bold

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Introduction

To set a UILabel's text to bold in iOS, assign a bold UIFont to its font property. The most common approach is UIFont.boldSystemFont(ofSize:) for the system bold font, or UIFont.systemFont(ofSize:weight:) for specific weights like .semibold or .heavy. For dynamic type support, use UIFont.preferredFont(forTextStyle:) with a bold text style. For partial bold text (bolding only certain words), use NSAttributedString with a bold font attribute applied to specific ranges.

System Bold Font

swift
1let label = UILabel()
2label.text = "Hello, World!"
3
4// System bold font at specific size
5label.font = UIFont.boldSystemFont(ofSize: 17)
6
7// Equivalent using weight parameter
8label.font = UIFont.systemFont(ofSize: 17, weight: .bold)

Available Font Weights

swift
1// From lightest to heaviest
2label.font = UIFont.systemFont(ofSize: 17, weight: .ultraLight)
3label.font = UIFont.systemFont(ofSize: 17, weight: .thin)
4label.font = UIFont.systemFont(ofSize: 17, weight: .light)
5label.font = UIFont.systemFont(ofSize: 17, weight: .regular)
6label.font = UIFont.systemFont(ofSize: 17, weight: .medium)
7label.font = UIFont.systemFont(ofSize: 17, weight: .semibold)
8label.font = UIFont.systemFont(ofSize: 17, weight: .bold)
9label.font = UIFont.systemFont(ofSize: 17, weight: .heavy)
10label.font = UIFont.systemFont(ofSize: 17, weight: .black)

Dynamic Type (Accessibility-Friendly)

swift
1// Automatically scales with user's text size preference
2label.font = UIFont.preferredFont(forTextStyle: .headline)  // Bold by default
3label.adjustsFontForContentSizeCategory = true
4
5// Other bold text styles
6label.font = UIFont.preferredFont(forTextStyle: .title1)
7label.font = UIFont.preferredFont(forTextStyle: .title2)
8label.font = UIFont.preferredFont(forTextStyle: .title3)

Custom Font with Dynamic Type Scaling

swift
1// Bold system font that scales with Dynamic Type
2let boldFont = UIFont.boldSystemFont(ofSize: 17)
3label.font = UIFontMetrics(forTextStyle: .body).scaledFont(for: boldFont)
4label.adjustsFontForContentSizeCategory = true

Custom Bold Fonts

swift
1// Custom font by name
2label.font = UIFont(name: "Helvetica-Bold", size: 17)
3
4// List available font names for a family
5for name in UIFont.fontNames(forFamilyName: "Helvetica") {
6    print(name)
7    // Helvetica
8    // Helvetica-Bold
9    // Helvetica-BoldOblique
10    // Helvetica-Light
11    // Helvetica-LightOblique
12    // Helvetica-Oblique
13}

Bold Variant of Any Font

swift
1extension UIFont {
2    var bold: UIFont {
3        guard let descriptor = fontDescriptor.withSymbolicTraits(.traitBold) else {
4            return self
5        }
6        return UIFont(descriptor: descriptor, size: 0)  // size 0 = keep original size
7    }
8
9    var italic: UIFont {
10        guard let descriptor = fontDescriptor.withSymbolicTraits(.traitItalic) else {
11            return self
12        }
13        return UIFont(descriptor: descriptor, size: 0)
14    }
15
16    var boldItalic: UIFont {
17        guard let descriptor = fontDescriptor.withSymbolicTraits([.traitBold, .traitItalic]) else {
18            return self
19        }
20        return UIFont(descriptor: descriptor, size: 0)
21    }
22}
23
24// Usage
25let regularFont = UIFont.systemFont(ofSize: 17)
26label.font = regularFont.bold
27label.font = regularFont.boldItalic

Partial Bold with NSAttributedString

Bold only specific words in a label:

swift
1let fullText = "Welcome to Swift programming"
2let boldText = "Swift"
3
4let attributedString = NSMutableAttributedString(string: fullText)
5let boldFont = UIFont.boldSystemFont(ofSize: 17)
6let regularFont = UIFont.systemFont(ofSize: 17)
7
8// Apply regular font to all
9attributedString.addAttribute(.font, value: regularFont,
10                               range: NSRange(location: 0, length: fullText.count))
11
12// Apply bold to specific range
13if let range = fullText.range(of: boldText) {
14    let nsRange = NSRange(range, in: fullText)
15    attributedString.addAttribute(.font, value: boldFont, range: nsRange)
16}
17
18label.attributedText = attributedString

Helper for Bold Substrings

swift
1extension NSMutableAttributedString {
2    func bold(_ text: String, fontSize: CGFloat = 17) -> NSMutableAttributedString {
3        let boldFont = UIFont.boldSystemFont(ofSize: fontSize)
4        let range = (self.string as NSString).range(of: text)
5        if range.location != NSNotFound {
6            addAttribute(.font, value: boldFont, range: range)
7        }
8        return self
9    }
10}
11
12// Usage
13let attributed = NSMutableAttributedString(string: "Hello World")
14    .bold("World")
15label.attributedText = attributed

Interface Builder / Storyboard

To set bold in Interface Builder:

  1. Select the UILabel in the storyboard
  2. Open the Attributes Inspector
  3. Click the Font field (the "T" icon)
  4. Change Style from "Regular" to "Bold"
  5. Or select a specific weight like "Semibold" or "Heavy"

SwiftUI Equivalent

swift
1// SwiftUI uses .bold() modifier
2Text("Hello, World!")
3    .bold()
4
5// Or font weight
6Text("Hello, World!")
7    .fontWeight(.bold)
8
9// Specific weights
10Text("Hello, World!")
11    .fontWeight(.semibold)
12
13// System font with weight
14Text("Hello, World!")
15    .font(.system(size: 17, weight: .bold))

Common Pitfalls

  • Using UIFont(name:size:) with a wrong font name: If the font name is incorrect (e.g., "Helvetica Bold" instead of "Helvetica-Bold"), the initializer returns nil and the label falls back to the default font. Print UIFont.fontNames(forFamilyName:) to find the exact name.
  • Not enabling adjustsFontForContentSizeCategory: Using preferredFont(forTextStyle:) without setting adjustsFontForContentSizeCategory = true means the font size is set once and never updates when the user changes their text size preference in Settings.
  • Applying attributedText after setting text: Setting label.text clears attributedText, and vice versa. If you set attributedText and then set text, the attributed formatting is lost. Use one or the other, not both.
  • Forgetting size: 0 in UIFont(descriptor:size:): When creating a font from a descriptor (e.g., adding bold trait), passing size: 0 preserves the original font size. Passing a specific size overrides it, which may cause unexpected size changes.
  • Hardcoding font sizes instead of using Dynamic Type: Hardcoded sizes like UIFont.boldSystemFont(ofSize: 17) do not respond to the user's accessibility text size settings. Use UIFontMetrics.scaledFont(for:) to make custom bold fonts support Dynamic Type.

Summary

  • Use UIFont.boldSystemFont(ofSize:) or .systemFont(ofSize:weight: .bold) for bold system fonts
  • Use UIFont.preferredFont(forTextStyle: .headline) with adjustsFontForContentSizeCategory = true for Dynamic Type support
  • Use fontDescriptor.withSymbolicTraits(.traitBold) to get the bold variant of any font
  • Use NSAttributedString with .font attribute to bold specific words within a label
  • In SwiftUI, use .bold() or .fontWeight(.bold) modifiers
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