Fitts's Law

The law

The time it takes to point at a target grows with the distance to the target and shrinks as the target gets bigger.

Movement time depends on the ratio of distance to width, not on either alone. A target twice as far away and twice as wide takes about the same time to hit.

Where it comes from

Paul Fitts published the finding in The information capacity of the human motor system in controlling the amplitude of movement, in the Journal of Experimental Psychology in 1954. Participants tapped back and forth between two metal plates with a stylus as fast as they could while staying accurate. Fitts varied the distance between the plates and their width.

He described each task by an index of difficulty, the base-2 logarithm of twice the distance divided by the target width, and found that movement time rose linearly with it. Later work in human-computer interaction, notably I. Scott MacKenzie's 1992 review, refined the formula and showed that it holds for mice, styluses and other pointing devices.

What it means for an interface

• Make frequent targets big. Primary buttons, list rows and form fields should have generous hit areas, not just generous visuals. The clickable area is what counts.
• Put targets near where the pointer already is. A button next to the field the person just filled in is faster to reach than one at the other side of the screen.
• Screen edges are large targets for a mouse. The pointer stops at the edge of the screen, so a target there cannot be overshot. That is part of why menu bars at the top of the screen and corner targets are fast with a mouse.
• Keep destructive actions apart. The same law works in reverse: a small, distant target is hard to hit by accident. Do not place "Delete" right beside "Save".

Examples

Form submission. A submit button directly under the last field, full width on a phone, is fast to reach. A small text link far to the right of the form takes longer to reach and is easier to miss.

Touch targets. On a touch screen the finger is the pointer and is much less precise than a mouse. Platform guidelines set minimum target sizes for this reason: Apple recommends at least 44 by 44 points, and Material Design recommends at least 48 by 48 density-independent pixels.

Context menus. A menu that opens at the pointer puts every option a short distance away, which is why right-click menus and long-press menus are quick to use.

Where it is misapplied

• Assuming edges help on touch. A finger does not stop at the screen edge the way a mouse pointer does, and edges on phones are often taken by system gestures. The edge advantage is a mouse effect.
• Counting visual size instead of hit area. A large icon with a small clickable area gains nothing. The target is whatever responds to the click or tap.
• Ignoring the time to find the target. The law models the movement once a person knows where to point. A large button that people cannot find or do not recognize is still slow.
• Making everything big. If every element is enlarged, distances grow too. Size helps most when it is given to the actions people take most often.

Checklist

• Primary and frequent actions have large hit areas, not only large visuals.
• Touch targets meet the platform's minimum size.
• The next action sits close to where the previous one ended.
• Destructive actions are separated from frequent ones.
• Pointer-only tricks such as edge placement are not relied on for touch layouts.

Related

• Hick's Law: the time to decide what to point at.
• Law of Proximity: placing related controls close together serves both grouping and reach.