Fitts's Law
A predictive model of human movement stating that the time to reach a target is a function of the distance to the target and its size, meaning larger and closer targets are faster to click.
What Is Fitts's Law?
Fitts's Law, formulated by psychologist Paul Fitts in 1954, mathematically describes the speed-accuracy tradeoff in aimed movements. The time to acquire a target is proportional to the logarithm of the distance divided by the target width. Translated to interfaces: bigger buttons are easier to click, closer buttons are faster to reach, and screen edges have effectively infinite size because the cursor cannot overshoot them. This 70-year-old motor movement model remains one of the most reliable predictors of click speed and error rates in digital interfaces.
Also Known As
- UX and design: "target size principle," "click target sizing"
- Product and engineering: "hit area sizing," "touch target guidelines"
- Marketing and growth: "CTA button sizing best practices"
- Mobile teams: "tap target specification" (Apple HIG minimum 44pt, Material Design 48dp)
How It Works
Imagine a SaaS pricing page with three plan cards. The "Start Free Trial" button on each card is 120 pixels wide and positioned 600 pixels below the pricing headline. A PM proposes shrinking it to 80 pixels to "look more elegant" and moving it below a 300-word feature list. Fitts's Law predicts the motor cost will jump: the index of difficulty roughly doubles when you halve the width and double the distance. In practice, click-through on that card drops because users who had already decided to convert are now fighting the interface to complete the action.
Best Practices
- Make primary CTAs at least 44x44 pixels on mobile and 32x32 on desktop; anything smaller punishes users with motor imprecision.
- Anchor persistent CTAs to screen edges (sticky footers, bottom navigation) to exploit infinite-width targets.
- Keep the primary action spatially close to the content that motivates it — the button should live next to the pricing, not below a long specification list.
- Do not put destructive and constructive actions adjacent at the same size; separate them or change weights to prevent accidental clicks.
Common Mistakes
- Using icon-only buttons at 24x24 pixels on mobile because they "look cleaner," then wondering why mobile conversion lags desktop.
- Placing the primary CTA far below the fold when a sticky or repeated CTA would shorten the motor path for decided users.
Industry Context
SaaS and B2B: trial and demo CTAs are high-value but often buried in marketing copy; moving them into sticky headers typically lifts form starts. Ecommerce and DTC: "Add to Cart" and "Buy Now" buttons benefit enormously from thumb-zone anchoring on mobile PDPs. Lead generation: form submit buttons should be at least as wide as the form fields themselves — narrower submits suppress completion rates.
The Behavioral Science Connection
Fitts's Law quantifies physical friction, which compounds with cognitive friction. Each millisecond of motor effort is a moment the user can reconsider, get distracted, or abandon. Reducing physical effort keeps decided users on the conversion path before second-guessing sets in.
Key Takeaway
Make primary actions big, close, and edge-anchored; every pixel of distance and every missing millimeter of width is a tax on conversion.