---
name: Fitts's law
slug: fitts-law
category: interaction
status: published
created: 2026-08-21T00:00:00.000Z
modified: 2026-08-21T00:00:00.000Z
definition: The rule that the time to hit a target grows with its distance and
  shrinks with its size, which is why screen edges and corners are so easy to
  reach.
aliases:
  - name: law of target acquisition
    source: community
  - name: index of difficulty
    source: community
  - name: magic corners
    source: community
  - name: target acquisition
    source: community
tags:
  - perception
  - pointer
relations:
  contrastWith:
    - touch-target-size
  variantOf: []
  partOf: []
  seeAlso:
    - thumb-zone
    - target-spacing
implementations: []
sources:
  - title: "Smashing Magazine: accessible tap target sizes"
    url: https://www.smashingmagazine.com/2023/04/accessible-tap-target-sizes-rage-taps-clicks/
  - title: "Understanding SC 2.5.8: Target Size (Minimum)"
    url: https://www.w3.org/WAI/WCAG22/Understanding/target-size-minimum.html
demo: inline
exhibit: false
useWhen: arguing about how big or how near a control should be
---

Paul Fitts published the model in 1954, about people moving a stylus between metal plates,
and it has survived every input device invented since. The claim is that pointing is a
trade of speed against accuracy, and that the difficulty of a move can be written as a
single number: the index of difficulty, roughly the base-two logarithm of twice the
distance divided by the width of the target. Movement time is then a straight line in that
number, with two constants that belong to the device and the person rather than to the
interface. The units are bits, because the model is really about how much information the
hand has to resolve to land on the thing.

Two consequences are worth carrying around, and they are not symmetric. Distance is inside
a logarithm, so halving it saves you less than you would guess, while width is also inside
the logarithm but is the term you usually have the most freedom over. That is why "make it
bigger" beats "move it closer" as advice, and why a control used constantly earns its
extra pixels. It also explains something people notice without naming: a small target you
have to travel to is disproportionately annoying, because the two penalties compound
inside the same expression.

The famous corollary is about edges. A pointer stops at the boundary of the screen, so a
control flush against an edge behaves as though it extends past it forever: the effective
width in the direction of travel is unbounded, the index of difficulty collapses, and you
can throw the pointer at it without aiming. A corner is two edges at once, which is why the
Start button and the Apple menu sit where they do, and why the macOS menu bar being welded
to the top of the screen has always been faster to hit than a menu bar inside a window a
few pixels below it. The web mostly forfeits this: a browser page has margins, and a
sticky bar with padding around it is an edge target that is not touching the edge.

The law is a model of aimed movement, not a law of usability, and it says nothing about
whether people can find the control or understand it. It also assumes an approach from a
known starting point, which is why the answer to "where should this button go" is usually
"near whatever the reader was just doing" rather than "in the middle". On touch there is no
travel to measure at all, and the size half of the trade takes over completely, which is
where target size and its minimums come back into it.