UX Glossary · F
Fitts's Law
Also: Fitts' Law
The time to hit a target depends on its distance and its size: near and large is fast, far and small is slow.
Fitts’s Law states that the time to move to a target is a function of the distance to it and its size. Paul Fitts published it in 1954, based on experiments with physical tapping tasks, and it transferred to pointing devices intact.
The relationship is logarithmic, not linear: doubling a small target’s size helps a lot, while doubling an already large one helps very little.
What follows for interfaces
Make important targets bigger. The primary action should not be the same size as everything around it.
Reduce distance. A control near where the user’s attention already is beats one in a distant corner. This is the argument for contextual toolbars and for putting an action beside the object it acts on.
Screen edges are effectively infinite. The pointer stops at the edge, so a target there cannot be overshot. This is why the macOS menu bar at the very top works well and why a menu one pixel below the edge is measurably worse.
Destructive actions should be harder to hit. The law works in both directions: putting “Delete” adjacent to “Save” guarantees mistakes.
On touch
Fingers are imprecise and cover the target. WCAG 2.2 sets a minimum target size of 24 by 24 CSS pixels at level AA, and 44 by 44 at AAA. Apple and Google both recommend around 44 to 48 points as a practical floor.