Fitts's Law

What it is

Formalised by psychologist Paul Fitts in 1954 for hand movements to physical targets, the law has been robustly confirmed in decades of HCI research for both mouse-cursor and touch-based interactions. The formula (T = a + b · log2(D/W + 1)) predicts movement time given target distance D and width W.

In UX, the specifics of the equation matter less than the two implications: make important targets bigger, and make important targets closer to where the user already is. Every touch-target and CTA sizing decision is a Fitts's Law decision.

Why it matters

Missed taps and mis-clicks aren't user error — they're a design failure to respect the physics of the interaction. Small targets at screen edges cause frustration, rage-tapping and abandonment. Fitts's Law is why the top-of-screen hamburger menu is such a poor pattern on phones: users hold the device at the bottom, so top-of-screen targets are far away in thumb-travel distance.

When to use it

Any time you're sizing an interactive element or deciding where to place it. Touch targets, buttons, tabs, close controls, navigation, form fields, drag handles, mobile bottom-nav vs top-nav decisions, cursor-based menu placement, error dismissal patterns.

What it looks like in practice

WCAG 2.5.8 requires a minimum touch target of 24×24 CSS pixels; Apple's Human Interface Guidelines recommend 44×44 CSS pixels; Material Design recommends 48×48 dp. The higher recommendations aren't arbitrary — they're calibrated to real thumb widths and Fitts-predicted error rates.

The bottom edge of a mobile viewport is a Fitts sweet spot for primary actions — close to the thumb's rest position, so travel time is minimal. Screen corners are Fitts poison — small effective target area and far from the thumb. Sticky bottom bars work because they respect Fitts.

On desktop, the four corners of the screen are Fitts's 'infinite' targets — a mouse can't overshoot past a screen edge, so the effective target area is infinite. This is why the macOS menu bar sits at the top edge and Windows Start button sits at the corner.

What people get wrong

• Sizing touch targets to visual weight rather than tap area. A 12px close X might look elegant but consistently gets mis-tapped.
• Placing critical actions at the top of long mobile screens. The 'save' button at the top of a form the user is filling in from bottom to top is a Fitts's Law failure — the user has to scroll and reach.
• Clustering small icons close together in mobile toolbars — dense tap zones where any tap becomes ambiguous.
• Ignoring the compounding of Fitts and Hick — when a menu is both far away and has many items, both the movement time AND the decision time climb.

Learn next

Fitts's Law sits alongside Hick's Law (decision time grows with options), and both are foundational for tap-target and interaction sizing decisions. It shows up in every mobile UX audit and every accessibility conformance check against WCAG 2.5.8.

Reviewed 29 September 2026 · Editorial standards