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Fitts's Law

Time to hit a target depends on how far away it is and how big it is, which makes size and distance the two levers you control.

Definition

Fitts's Law models pointing as a speed-accuracy trade-off. Movement time increases with the distance to a target and decreases with the target's size along the axis of motion: MT = a + b·log₂(2D / W). The log term is the index of difficulty, measured in bits.

Established research. Originates in published research and has been replicated. The effect is real. Its size in your product still needs measuring.

On this page
  1. Definition
  2. One of the most robust findings in HCI
  3. Practical consequences
  4. Minimum target sizes in published guidance
  5. In practice
  6. Sources
  7. Continue from here

One of the most robust findings in HCI

Paul Fitts published the original work in 1954 using a stylus-tapping task. The relationship has since held across mice, trackpads, touchscreens, styluses, eye gaze and foot pedals. Scott MacKenzie's 1992 Shannon formulation is the version most commonly used today because it behaves better at low indices of difficulty.

For designers the useful reduction is simple: make important targets bigger, put them closer to where the pointer already is, and exploit the edges of the screen.

pointerdistance Dnarrow W: slowersame distance Dwide W: faster
Distance and width both feed the index of difficulty. Widening a target along the direction of travel helps more than widening it across that direction.

Practical consequences

  • Screen edges and corners behave as infinitely deep targets, because the pointer cannot overshoot past them. This is why the macOS menu bar at the top edge is faster than a menu bar inside a window.
  • The clickable area matters, not the visible area. A 16px icon inside a 44px hit area obeys the 44px.
  • Radio buttons and checkboxes should have their labels inside the target. Doing so often triples the effective width for free.
  • Contextual menus that open at the pointer beat fixed toolbars for repeated actions, because D approaches zero.
  • Destructive actions benefit from being small and far away, which is the law applied in reverse.

Minimum target sizes in published guidance

These are guidance figures from platform vendors and the WCAG standard, not outputs of the equation. They are floors, not targets to aim for.

SourceMinimumNotes
WCAG 2.2, Target Size (Minimum), AA24 × 24 CSS pxSuccess criterion 2.5.8. Exceptions exist for inline links and user-agent controls.
WCAG 2.1, Target Size (Enhanced), AAA44 × 44 CSS pxSuccess criterion 2.5.5.
Apple Human Interface Guidelines44 × 44 ptLong-standing iOS guidance.
Material Design48 × 48 dpWith at least 8dp of spacing between targets.

In practice

The bottom bar on mobile

Mobile app navigation

Primary navigation moved from the top of phone screens to the bottom because the thumb rests near the bottom edge. Distance dropped for the most frequent action in the app, and the bottom edge adds an effective backstop.

Full-row list targets

Settings and lists

Making the whole row tappable rather than only the chevron increases target width by an order of magnitude at zero visual cost, and removes the most common source of mis-taps in settings screens.

Sources

Where a source establishes something narrower than the popular reading of it, the note says so.

  • The information capacity of the human motor system in controlling the amplitude of movement

    P. M. Fitts, Journal of Experimental Psychology, 1954

  • Fitts' law as a research and design tool in human-computer interaction

    I. S. MacKenzie, Human-Computer Interaction, 1992

    Introduces the Shannon formulation used in most modern HCI work.

  • Understanding Success Criterion 2.5.8: Target Size (Minimum)

    W3C, WCAG 2.2, 2023

Continue from here

Each link says what the connection is, so you can tell a principle from an alternative from a thing people mix this up with.

Applies when you are designing

A Data TableA Mobile AppAn Enterprise WorkflowNavigation

Explains

The mechanism underneath these entries.

  • Target SizeAccessibility

    The WCAG minimum is the accessibility floor under an effect that is really about distance and size together.

Applied through

Where this shows up as a concrete interface decision.

Check your work

Review passes that test whether this was done properly.

Related concept

Connected closely enough to change how you apply this.