Fitts's law

Fitts's law is a model of pointing. It predicts how long it takes to move a finger, mouse or other pointer to a target, based on the distance to the target and the target's width. The psychologist Paul Fitts described it in a 1954 paper that applied information theory to the human motor system. The relationship is logarithmic, not linear: doubling the distance does not double the time, because people move fast at first and slow down as they approach the target.

Why it matters

Every click and tap is a small pointing task, and they add up. Small or far-away targets are slower to reach and easier to miss, which turns into mis-taps, accidental actions and frustration, especially on phones and for people with limited dexterity. NN/g's article on Fitts's law summarizes it as: "The movement time to a target depends on the size of the target and the distance to the target."

Platform guidelines turn this into numbers. Apple's Human Interface Guidelines list a default control size of 44x44 pt on iOS and iPadOS, with 28x28 pt as the minimum. WCAG 2.2 success criterion 2.5.8 (Level AA) requires pointer targets of "at least 24 by 24 CSS pixels", with listed exceptions such as adequate spacing.

How to apply it

• Do make frequent and primary actions large. Give icon buttons a generous hit area, and add a text label, which both clarifies the action and enlarges the target.
• Do put related controls close to where the user already is. In a chat assistant, keep "Send" next to the input, and put "Copy" and "Retry" right under the response they act on, not in a distant toolbar.
• Do use screen edges and corners for pointer-driven desktop UI. A pointer stops at the edge, so targets there behave as if they were infinitely deep in one direction. NN/g cites the macOS menu bar at the top of the screen as an example.
• Do leave space between adjacent targets. Apple's guidelines treat spacing as being as important as size.
• Don't place destructive actions right next to common ones. A large, close "Delete" next to "Save" makes the wrong action easy too.
• Don't rely on tiny inline links or close icons for important actions on touch screens.

Common mistakes

• Treating Fitts's law as only "make buttons bigger". Distance matters as much as size: a big button at the far side of the screen can still be slow.
• Making the visible element small while the tappable area is the same small size. The hit area can be larger than the visual.
• Applying edge and corner tricks to touch screens. The infinite-edge benefit comes from a pointer that stops at the screen boundary. On a phone, edges are often harder to reach with a thumb.
• Forgetting that the law models pointing time only. It says nothing about how long a person spends deciding what to click; that is closer to Hick's law.