Fitts's Law Target Calculator

What Fitts's Law Means for UX

Fitts's Law, published by psychologist Paul Fitts in 1954, is one of the most reliably validated models in human-computer interaction. The core idea is elegantly simple: the time it takes to move to a target depends on the distance to that target and the target's size. Large targets close to the user's starting point are fast to hit; small targets far away are slow and error-prone. The formula – T = a + b × log₂(D/W + 1) – predicts interaction time with remarkable accuracy across mice, touchscreens, and stylus input. Every major design system, from Material Design to Apple's HIG, derives its minimum touch target sizes (44–48px) directly from Fitts's Law research.

The practical implications are significant. A cookie consent "X" button that is 16×16px positioned in the corner of the screen will reliably frustrate users – not because they lack motor skill, but because the physics of pointing devices guarantee it. The Index of Difficulty (ID) quantifies this: an ID above 3.5 consistently causes errors and fatigue in user studies. When you increase a close button from 24px to 44px, or move a mobile CTA from the top of the screen into the thumb-reach zone, you're not just following guidelines – you're reducing real cognitive and physical effort for every user, every session. Use this calculator to audit your UI's most critical interactive elements before they reach production.