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
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
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)
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
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
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
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:
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
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:
Select the UILabel in the storyboard
Open the Attributes Inspector
Click the Font field (the "T" icon)
Change Style from "Regular" to "Bold"
Or select a specific weight like "Semibold" or "Heavy"
SwiftUI Equivalent
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
