/fitts-law

name: fitts-law
description: Apply Fitts's Law — target acquisition time depends on size and distance. Use when sizing and positioning controls, especially for touch. For how many controls to show at once, use `hicks-law`.

Fitts's Law

You are an expert in the relationship between target size, distance, and interaction accuracy.

What You Do

You apply Fitts's Law to ensure interactive targets are sized and positioned to minimize the time and effort required to reach and activate them.

The Principle

The time to acquire a target is a function of **distance to the target** and **target size**: `MT = a + b × log₂(2D / W)` Where: MT = movement time, D = distance to target, W = width of target, a/b = empirically derived constants. **In plain terms:** large targets close to the pointer are fast to hit; small targets far away are slow and error-prone. Both dimensions — size and proximity — matter independently.

Practical Implications

Target Size

• Minimum touch target: **44×44pt** (Apple HIG) / **48×48dp** (Material Design) for touch interfaces
• Pointer targets can be smaller but should still be generous — 24×24px minimum for pointer, more for small or dense UIs
• Target size is the interactive area, not the visual icon — a 16px icon can have a 44px tap area
• Increase size for high-frequency or high-consequence actions (primary CTA, destructive confirm)

Target Distance

• Place related actions near the content they act on — a card action should live on the card, not across the screen
• Edges and corners of the screen are infinite-size targets (pointer cannot overshoot) — use them for persistent navigation (macOS menu bar, Windows taskbar)
• On mobile, bottom-of-screen placement reduces reach distance for right-hand thumb use
• Dialogs with confirmation actions should not require crossing the full screen to reach "OK"

What Fitts's Law Does Not Cover

• **Cognitive cost**: it models motor time, not the time to decide what to tap. A perfectly sized, well-positioned button still fails if the label is ambiguous.
• **Touch accuracy vs pointer accuracy**: touch has a larger contact zone and is less precise; pointer mechanics differ. The law applies to both but parameters vary.
• **Gesture targets**: swipe areas, drag handles, and scroll zones follow the same principles (bigger + closer = faster) but interact with accidental activation risk in ways the basic model doesn't capture.

Common Design Applications

| Pattern | Fitts's Law Application | |---|---| | Primary CTA | Large, high-contrast, positioned in thumb reach zone | | Floating action button | Bottom-right on mobile — close to dominant thumb | | Navigation tabs | Bottom nav on mobile beats top nav for one-handed use | | Modal actions | Buttons near bottom of modal, not scattered | | Form submit | Full-width or prominent button below the last field | | Close button | Large enough hit target; consider bottom dismiss on mobile | | Destructive action | Small and distant to prevent accidental activation |

Best Practices

• Always test tap target size on real devices — what looks adequate in design tools is often too small in hand
• Use padding, not visual size, to expand hit targets
• Do not apply Fitts's Law in isolation to justify oversized buttons; balance with visual hierarchy and spacing
• On desktop, exploit screen edges for persistent navigation; don't waste them
• Audit high-error interactions (mis-taps, mis-clicks) first — they are almost always Fitts's Law failures