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Design · Frontend / AGENT SKILL
fitts-law
0
Apply Fitts's Law to size and position controls, especially for touch interfaces.
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`.
Understand the trade-offs.
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Original instructions from the publisher’s SKILL.md
# 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