Dynamic type scaling

Dynamic Type and Font Scaling

Instructions

Users can set their system font 1.5–2x larger than default. Apps that clamp or ignore this scale are the #1 source of real-world accessibility complaints.

1. iOS — Dynamic Type

Use text styles, not point sizes.

UIKit:

let label = UILabel()
label.font = UIFont.preferredFont(forTextStyle: .body)
label.adjustsFontForContentSizeCategory = true
label.numberOfLines = 0

SwiftUI:

Text("Welcome")
    .font(.title2)
    .dynamicTypeSize(.small ... .accessibility5) // cap only if truly necessary

• Prefer .body, .headline, .callout, .footnote over .system(size:).
• adjustsFontForContentSizeCategory = true on every label, text field, button title.
• Use UIFontMetrics(forTextStyle:).scaledValue(for:) for layout metrics that should scale too (icon size, row height).

2. Android Views — sp

• All text sizes in sp (scale-independent pixels), all other dimensions in dp.
• Never use px for text. Never use dp for text except inside a WebView or Canvas where scaling is applied manually.

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/welcome"
    android:textAppearance="?attr/textAppearanceBodyLarge" />

• Prefer Material type scale (?attr/textAppearanceBodyLarge, titleMedium, etc.) so sizes come from the theme.

3. Jetpack Compose — fontScale

Compose text already scales with LocalDensity.fontScale. Don't fight it.

Text(
    text = "Welcome",
    style = MaterialTheme.typography.titleLarge,
)

If you accept a TextUnit, use sp:

Text("Subtitle", fontSize = 14.sp) // NOT .dp

To test different scales:

CompositionLocalProvider(LocalDensity provides Density(
    density = LocalDensity.current.density,
    fontScale = 1.5f
)) { App() }

Preview helper:

@Preview(fontScale = 1.5f, showBackground = true)
@Composable fun PreviewLargeFont() { /* ... */ }

4. Flutter — TextScaler

Flutter 3.16+ uses TextScaler (replacing deprecated textScaleFactor).

final scaler = MediaQuery.textScalerOf(context);
final scaled = scaler.scale(16.0); // use for icon sizes, row heights

Text(
  'Welcome',
  style: Theme.of(context).textTheme.titleLarge, // scales automatically
)

Never force TextScaler.linear(1.0) globally. If you must clamp:

MediaQuery(
  data: MediaQuery.of(context).copyWith(
    textScaler: MediaQuery.textScalerOf(context)
        .clamp(minScaleFactor: 1.0, maxScaleFactor: 1.8),
  ),
  child: child,
)

Wrap long strings in Flexible/Expanded so they wrap instead of overflow.

5. React Native — allowFontScaling

Default is true — keep it.

<Text style={{ fontSize: 16, lineHeight: 22 }}>
  Welcome
</Text>

Never do:

<Text allowFontScaling={false}>Welcome</Text>   // BAD globally
Text.defaultProps = { allowFontScaling: false } // BAD globally

If you must cap (e.g., for pixel-perfect brand headers):

<Text maxFontSizeMultiplier={1.8}>Brand headline</Text>

For icons inside buttons, scale manually using PixelRatio.getFontScale():

import { PixelRatio } from 'react-native';
const iconSize = 20 * Math.min(PixelRatio.getFontScale(), 1.8);

6. Layout Rules for Scalable Text

• Never fix a text container's height in dp/pt/px. Use wrap_content / intrinsic size.
• Use minimumScaleFactor sparingly — shrinking text fails WCAG 1.4.4.
• Prefer vertical stacking over horizontal for label-value pairs when font scale > 1.3.
• Truncating is acceptable only for preview text that has a "see more" affordance.

7. Testing Large Sizes

• iOS Simulator: Features → Toggle Accessibility Inspector → Dynamic Type slider; or Settings → Accessibility → Display & Text Size → Larger Text.
• Android Emulator: Settings → Display → Display size and text → Font size (largest). Also test Display size (density scaling).
• Flutter: wrap root with MediaQuery(..., textScaler: TextScaler.linear(2.0)) in a debug build.
• RN: enable system large text; also write Jest snapshot with PixelRatio mocked.

8. Common Pitfalls

• Clamping font scale to 1.0 "because the design breaks". Fix the design.
• Using fontSize for numerical metrics (avatar diameter) — use text-style-relative sizing.
• SingleLine = true on labels that contain translated content — truncation.
• Hard-coded row heights in lists; switch to intrinsic + padding.
• Passing dp or px to a TextView via code.

Checklist

• All text uses scaled units (sp, .preferredFont, TextScaler, RN default).
• No allowFontScaling={false} globally; caps justified per-case.
• Screens tested at largest accessible Dynamic Type / font scale.
• Icons and row heights scale proportionally where appropriate.
• No horizontal clipping / truncation of essential content at 200%.
• Previews / snapshots exist at an enlarged font scale (1.5x+).