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fitts-law

Use this skill whenever the question involves how easy it is to *hit* an interactive target — buttons, links, form controls, drag handles, scrollbars, mobile tap targets, anything the user must move a pointer or finger to. Trigger when sizing buttons, designing toolbars, picking a hit-area for icon buttons, designing for touch screens, debugging "users keep mis-clicking," or arguing about button density vs. usability. Trigger when the user mentions touch targets, target size, "fat fingers," pointer accuracy, edge gestures, or accessibility around motor impairments. Routes to sub-aspect skills for touch targets, screen edges, and pointer acceleration.

plugins/HDeibler/universal-design-principles/plugins/interaction-and-control-principles/skills/fitts-law/SKILL.md

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# Fitts's Law

Fitts's Law is the foundational quantitative model of how long it takes to acquire (point to) a target with a directed movement. The classic formulation:

```
T = a + b · log₂(D / W + 1)
```

Where T is movement time, D is the distance from the starting position to the target, W is the target's width along the axis of motion, and a, b are device-and-context constants. The logarithmic relationship is the core insight: doubling the size of a target reduces movement time by a constant amount; halving the distance to a target also reduces it by a constant amount; targets at the edges of the screen are effectively infinite in one dimension because the cursor stops there.

## Definition (in our own words)

The time to move a cursor or finger to a target is determined by two things: how *far* the target is from where you start, and how *big* the target is. Big nearby targets are fastest; small distant targets are slowest. Edges and corners of the screen are exceptional: the cursor (or finger) cannot overshoot, so they behave as effectively infinite-sized targets along that one axis.