The bigger and closer a target, the faster a user can reach it. Most interaction design decisions follow from this.

Fitts's Law is a predictive model of human motor movement, first published by psychologist Paul Fitts in 1954. In plain terms: the time it takes to move a pointer or finger to a target depends on two things — the target's size and its distance from the starting point. Larger targets and closer targets are faster to reach.

The relationship is logarithmic, not linear. Doubling a button's size doesn't halve the time to hit it — but it does reduce it, and the effect is consistent and measurable. It's one of the few psychological principles in UX that produces directly quantifiable predictions.

Fitts's Law has a short list of high-impact implications:

• Primary action buttons should be the largest interactive element on a screen. If the CTA is smaller than a nearby secondary or destructive action, that's a Fitts problem.
• Touch targets on mobile need to be larger than they look. Apple's Human Interface Guidelines recommend a minimum of 44×44pt; Google Material Design specifies 48×48dp. Both are grounded in motor accuracy at smaller sizes.
• Screen edges and corners are valuable real estate. A target placed against the edge of a screen is effectively infinite in one dimension — you can't overshoot it. This is why the macOS menu bar sits at the top edge of the screen rather than the top of the application window.
• Destructive actions should be small and distant from primary actions. Keeping delete and cancel controls physically separated from the main flow makes accidental activation harder — using Fitts's Law intentionally.

Fitts's Law is part of why bottom navigation bars outperform top navigation on mobile. The bottom of the screen is within easy thumb reach — lower distance, lower error rate, faster access.

It also explains the value of contextual actions. An action placed inline, adjacent to the content it affects, is faster to reach than the same action buried in a toolbar at the top of the page. In complex B2B products with many actions per row or card, proximity to the content being acted on meaningfully reduces the time and cognitive effort per interaction.

Fitts's Law describes pointing efficiency in isolation. It doesn't account for:

• Hesitation. A large button with an ambiguous label is still slow — users pause before committing to an unclear action. Size helps after the decision is made; clarity helps before.
• Attention. Users can't hit a target they haven't noticed. Visual Hierarchy determines whether the target registers at all.
• Touch occlusion. On mobile, the finger covers what you're about to tap — a problem size alone doesn't fully solve. Adequate spacing between targets matters as much as target size.

Fitts's Law is most useful as an audit tool for existing designs — identifying targets that are measurably too small or poorly positioned — rather than a standalone design framework.