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`.
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You are an expert in the relationship between target size, distance, and interaction accuracy.
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 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.
| 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 |
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概要と使いどころ
Design an A/B experiment — hypothesis, variants, primary metric, and sample size. Use when a change can be measured quantitatively at scale. For observing behaviour qualitatively, use `test-scenario`.
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Audit an existing interface against WCAG, producing findings with severity ratings and remediation steps. Use when you have a design or build to assess now. Not for planning future sessions with assistive-technology users — use `accessibility-test-plan` (prototyping-testing).
日本語の概要は準備中です。原文の説明を表示しています。
Plan accessibility testing — assistive technologies, participant criteria, WCAG coverage, and session protocol. Use when scheduling testing with real AT users. Not for evaluating a design yourself — use `accessibility-audit` (design-systems).
日本語の概要は準備中です。原文の説明を表示しています。
Apply the Aesthetic-Usability Effect — polished, consistent interfaces are perceived as more usable and forgive minor friction. Use when justifying visual polish or diagnosing why a functional design tests badly. For emotional resonance specifically, use `interfaces-that-feel` (interaction-design).
日本語の概要は準備中です。原文の説明を表示しています。
Cluster many qualitative data points into themes and insight statements. Use when synthesising across multiple sessions or sources. For a single transcript use `summarize-interview`; for one segment's inner state use `empathy-map`.
日本語の概要は準備中です。原文の説明を表示しています。
Apply animation principles — easing, staging, follow-through — to one specific UI motion. Use when tuning how an animation feels. For product-wide duration and easing tokens use `motion-system` (design-systems); for a full interaction spec use `micro-interaction-spec`.
日本語の概要は準備中です。原文の説明を表示しています。