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Time to reach a target depends on distance and size.
stellae.design
Fitts's Law is a predictive model showing that movement time increases with target distance and decreases with target size. In UX, this means small or far-away interactive elements slow users down and cause errors — especially on touch devices.
The time to acquire a target is a function of the distance to and size of the target. Smaller and more distant targets take longer to select.
Click the start button, then hit each target as fast as you can. Larger, closer targets are faster to reach.
Large, well-placed primary action buttons
Key actions have generous click targets in thumb-friendly zones
Tiny icons with no padding as primary actions
Small targets far from the content they relate to
Fitts's Law isn't a cognitive bias — it's a measurable property of human motor control. Paul M. Fitts's 1954 model describes movement time as a function of distance (D) and target width (W), often expressed as ID ∝ log₂(2D/W). It remains one of the most successful predictive models in ergonomics and HCI, with decades of replication. Platform guidelines from Apple (44×44pt minimum), Google Material (48×48dp), and WCAG 2.2 all operationalise this law.
The macOS dock sits at the screen edge, which effectively gives it infinite height as a target — the cursor cannot overshoot the edge. Apple's menu bar at the top of the screen uses the same principle. Both leverage Fitts's Law by making the most frequently accessed targets effectively infinitely tall in one dimension.
Mobile interfaces that place primary actions within the natural thumb reach zone (bottom-center of the screen for one-handed use) are applying Fitts's Law to touch interfaces. The effective distance from the resting thumb position to these targets is minimized, while the button sizes are generous. This is why bottom navigation bars and floating action buttons became standard mobile patterns.
Some desktop applications use icon-only toolbars with 16x16 pixel targets and no labels. Users must make precise mouse movements to hit these tiny targets, dramatically increasing error rates and task completion time. Despite looking clean, these designs violate Fitts's Law by minimizing target size while providing no distance advantage.
Right-click context menus appear directly at the cursor position, reducing the distance component of Fitts's Law to near zero. The menu items are full-width text labels providing generous target sizes. This combination of zero distance and large targets makes context menus one of the fastest interaction patterns available in desktop interfaces.
• Oversimplifying to 'make buttons huge' — the real problem is optimisation under constraints of density, aesthetics, hierarchy, and reachability.
• Applying mouse rules to touch or touch rules to XR gaze/gesture without testing the actual input modality.
• Ignoring thumb-zone research for one-handed mobile use.
| Check | Good Pattern | How to Test |
|---|---|---|
| Touch target sizing | All interactive elements meet minimum touch target sizes: 44x44pt (iOS), 48x48dp (Android), or equivalent padding for web. | Use a design tool's measurement feature or browser dev tools to verify the clickable/tappable area of every interactive element. Automated accessibility scanners can flag undersized targets. |
| Primary action proximity | The most frequently used actions are positioned closest to where the user's cursor or thumb naturally rests. | Create a heatmap of click/tap locations from analytics. Verify that the highest-frequency actions correspond to the easiest-to-reach screen positions. |
| Target spacing and error prevention | Destructive actions are adequately spaced from frequent actions, and sufficient padding between targets prevents mis-taps. | Audit all button groups for adjacent destructive + constructive actions. Test on mobile devices to verify that rapid tapping doesn't accidentally trigger adjacent targets. |
Sometimes smaller targets are unavoidable: dense data tools, maps, timelines, or professional dashboards. Compensate with zoom, keyboard shortcuts, snapping, or progressive disclosure. WCAG includes explicit exceptions where size/spacing are constrained by the information being conveyed.
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