Fitts's Law
A predictive model showing that the time to acquire a target is a function of distance and size. Essential for button placement and sizing.
What is it?
Fitts's Law is a predictive model of human movement, originally formulated by psychologist Paul Fitts in 1954. It states that the time required to move to a target is a function of two things: the distance to the target, and the size of the target. Larger targets that are closer are faster to acquire. Smaller targets that are farther away take significantly longer.
Why it matters
In digital interfaces, Fitts's Law governs how quickly users can click buttons, tap targets, and interact with controls. A small button in the corner of a screen takes measurably longer to click than a large button in the center. This translates directly into task completion time, error rates, and user satisfaction.
Best Practices
- Make primary action buttons large. On mobile, a minimum 44×44px touch target (Apple HIG standard). Larger is better for high-frequency actions.
- Place primary actions close to where the user's cursor or thumb naturally rests — bottom-right on desktop, bottom center on mobile (thumb zone).
- Screen edges and corners have infinite size in one direction — a menu attached to the left edge is always just as wide as the screen minus current cursor position.
- Avoid placing frequently-used actions far from the user's starting position. Toolbar actions used constantly should be close together.
- On mobile, design for one-handed use. The thumb naturally reaches the bottom third of the screen. Critical actions belong there.
- Increase click area beyond the visual boundary. A small icon can have a much larger invisible click zone — common practice in mobile apps.
- For destructive actions, keep them small and far. Increasing distance and reducing size adds a deliberate friction that prevents accidents.
- Stacked actions in a list should have adequate spacing — too close together causes mis-taps.
Common Mistakes
- Small icon-only buttons with no expanded touch area on mobile.
- Placing the primary CTA button far from where the user's eye and cursor naturally land.
- Navigation items spaced too close together on mobile, causing constant mis-taps.
- Confirm/Cancel buttons that are small and adjacent — accidental cancels are common.
- Ignoring Fitts's Law in drag-and-drop interfaces — small drop targets are significantly harder to use.
Checklist
Research & Theory
Fitts's Original Paper (1954)
Paul Fitts published "The information capacity of the human motor system in controlling the amplitude of movement" in the Journal of Experimental Psychology.
Why it's relevant
The mathematical model (ID = log₂(2D/W)) predicts movement time precisely. In practice, the takeaway is: bigger + closer = faster.
Touch vs. Mouse (Parhi et al., 2006)
Research showed that touch targets need to be larger than mouse click targets because finger precision is lower than cursor precision.
Why it's relevant
The 44px minimum for iOS touch targets and 48dp for Android are directly derived from Fitts's Law applied to finger-based input.
Steering Law (Accot & Zhai, 1997)
An extension of Fitts's Law for paths through tunnels (like menus). The time to navigate a tunnel is proportional to its length and inversely to its width.
Why it's relevant
Cascading menus (long, narrow paths) are inherently hard to navigate under Fitts's principles. This is why they're largely abandoned in modern UX.
Real-World Examples
iOS
The home button, then touch ID, now Face ID — each iteration optimizes for zero movement to the most frequent action. Screen-edge gestures have infinite target size in one axis.
Figma
The toolbar is at the top edge — infinite target height downward. Panel handles on the right edge — infinite target width rightward. The placement is not aesthetic.
Linear
Command palette (Cmd+K) eliminates Fitts's Law entirely for power users — keyboard shortcuts have zero distance.