Fitts' Law

A predictive model of human movement that describes the time required to move to a target as a function of the target's size and distance from the starting point. Formulated by Paul Fitts in 1954, the law states that smaller and more distant targets take longer to acquire. In accessibility contexts, Fitts' Law is fundamental to understanding why people with motor impairments may struggle with small interface elements like buttons, links, and menu items — the combination of reduced motor precision and small targets dramatically increases task difficulty. The ISO 9241-9 standard uses Fitts' Law as the basis for evaluating pointing device performance. Accessibility guidelines recommending minimum target sizes (such as WCAG 2.5.5's 44×44 CSS pixel target size) are informed by the principles Fitts' Law describes.