Fitts's Law & Target Sizing

The plain-language idea

Fitts's Law models the time it takes to move a pointer (a mouse cursor, a finger, a thumb) to a target as a function of two things: the distance to the target and the target's size. Farther and smaller both make a target slower and harder to hit; closer and bigger both make it faster and easier. It's one of the oldest, most consistently reproduced findings in human-computer interaction, and it's the actual mechanism behind two pieces of UI advice that get repeated without their reasoning attached:

• Minimum touch-target sizes. Mobile guidelines commonly cite around 44×44px as a practical floor for a tappable target — worth knowing as the widely-cited number, not as an exact universal constant, since the underlying law cares about the actual distance and precision required, not a single magic pixel value.
• Placing frequent actions near existing attention/cursor position. A "Save" button that appears right where someone is already working costs less time-to-reach than the identical button pinned in a far corner — same size, different distance, different Fitts's Law cost.

See it in a real thing: screen edges are an infinite target

Close and large: short distance, generous size — the fastest, easiest target to hit.

BackNext

Fitts's Law has a special case that explains a specific, non-obvious pattern: a target pinned to a screen edge or corner behaves like it has infinite size in the dimension perpendicular to the edge, because the cursor physically cannot overshoot past the edge of the screen. Flinging the mouse hard toward the very top of the screen and stopping precisely on a 20px-tall menu bar is effortless — the edge does the stopping for you. The identical 20px target floating in the middle of the screen has no such backstop; overshoot in any direction and you miss. This is why OS menu bars, taskbars, and browser chrome are pinned to edges rather than floating, and why a floating action button is a meaningfully harder target than a bottom-edge-anchored one at the same visual size.

What people usually miss

Watch out

• "44×44px" is a widely-cited mobile guideline, not a precise universal law. State it as the commonly-referenced practical floor it is — inventing more precision than that (an exact number for every context, every input method) overstates what the guidance actually claims.
• Distance matters as much as size, and gets ignored more often. A perfectly-sized button that's far from where the user's attention already is still costs real time-to-reach — placement is not a cosmetic decision, it's the other half of the same equation as size.
• Screen edges and corners aren't just "convenient" for global controls — they're measurably easier targets, because the cursor can't overshoot past them. This is a real, mechanical reason for pinning system-level UI to edges, not a stylistic convention.
• Fitts's Law applies to touch and cursor input differently. Touch targets need to account for finger contact-area imprecision (and the target being partially occluded by the finger itself) on top of the same distance/size tradeoff, which is part of why touch-target minimums tend to be larger than comparable mouse-target minimums.

Why this matters in interviews

"Why should this button be bigger, or why should it move?" is a question that separates "it looks nicer" from a candidate who can name Fitts's Law and explain the actual mechanism — time-to-target as a function of distance and size — including the edge/corner special case, which is the detail that shows real understanding rather than a memorized guideline.