Fitts’s Law
Date: 2026-08-17
Time to hit a target depends on its size and how far away it is. It’s the most reliably predictive law in interface design, and on touch devices it’s the reason a small tap target is a genuine accessibility failure rather than a stylistic choice.
Fitts’s law states that the time to acquire a target is a function of the distance to it and its size:
Where D is distance to the target and W is its width along the axis of movement.
Working it
Taking a = 0.1s and b = 0.1s:
D (px) W (px) 2D/W log₂ T
200 48 8.3 3.06 0.41s
200 24 16.7 4.06 0.51s
200 12 33.3 5.06 0.61s
400 48 16.7 4.06 0.51s
100 48 4.2 2.06 0.31s
In plain terms: halving a target’s size costs about as much as doubling the distance to it. Both matter, and size is usually the one you control.
The practical consequences
Big targets are faster and more accurate. Not a preference — a measurable, replicated effect.
MINIMUM TOUCH TARGET
~44×44 CSS pixels is the common
guidance, with adequate spacing
between adjacent targets
[CHECK: current WCAG target-size success criterion and its exact minimum before citing a number as a requirement — this has changed between WCAG versions.]
Distance from where the finger already is matters as much as absolute position. On mobile, the bottom of the screen is near the thumb and the top is not — which is why primary actions belong at the bottom on touch and why a top-right “Add to basket” is a poor choice on a phone — Mobile Interaction Patterns.
The infinite targets
The genuinely useful corollary: screen edges have effectively infinite width, because the pointer stops there.
a button at the very edge
→ you cannot overshoot it
→ W is effectively infinite
→ T collapses
This is why desktop operating systems put menus at the very top or dock at the very edge, and why a sticky bar flush to the bottom of a mobile viewport is easier to hit than one with a margin beneath it.
Where it shows up commercially
- Add-to-basket buttons. Size and position, measurably
- Quantity steppers. Small
+and−controls next to each other is the canonical Fitts’s law failure — small targets, adjacent, with different consequences for mis-tapping - Close buttons on modals. Small, in a corner, often near something destructive
- Delete versus edit sitting adjacent, same size. Separate destructive actions from routine ones by distance, deliberately
- Checkout form controls, where a mis-tap costs a correction and correction costs abandonment — Form Design
Making the target bigger than it looks
The technique worth knowing: the clickable area doesn’t have to match the visible one.
.icon-button {
position: relative;
}
.icon-button::after {
content: "";
position: absolute;
inset: -12px; /* extends the hit area */
}A 20px icon can carry a 44px target without changing the design. This is the single most useful application of the law and it’s routinely missed on icon-only controls.
Limits
- It models pointing, not deciding. Choosing which target is Hick’s Law
- It assumes a single continuous movement. Scrolling to reach something is a different action
- Touch differs from mouse. Fingers occlude the target and have a much larger contact area than a cursor’s hotspot, so the effective precision is worse than the visual size suggests
- It says nothing about whether the target is findable. A large, fast-to-hit button nobody notices is not a success — Affordances and Signifiers