In ergonomics, Fitts' law is a model of human movement, predicting the time required to rapidly move from a starting position to a final target area, as a function of the distance to the target and the size of the target. Fitts' law is used to model the act of pointing, both in the real world, for example, with a hand or finger and on computers, for example, with a mouse. It was published by Paul Fitts in 1954.

Mathematically, Fitts' law has been formulated in several different ways. One common form is the Shannon formulation (proposed by Scott MacKenzie, and named for its resemblance to the Shannon-Hartley theorem) for movement along a single dimension:

where

T is the average time taken to complete the movement. (Traditionally, researchers have used the symbol MT for this, to mean movement time.)

a and b are empirical constants, and can be determined by fitting a straight line to measured data.

D is the distance from the starting point to the center of the target. (Traditionally, researchers have used the symbol A for this, to mean the amplitude of the movement.)

W is the width of the target measured along the axis of motion. W can also be thought of as the allowed error tolerance in the final position, since the final point of the motion must fall within ± W/2 of the target's centre.

From the equation, we see a speed-accuracy tradeoff associated with pointing, whereby targets that are smaller and/or further away require more time to acquire.