# Fitts law

> (In HCI) Fitts' law.

(In <HCI>) Fitts' law.

$$
T = a + b \log_2\!\left(1 + \frac{D}{W}\right)
$$

Meaning of each variable:

- $T$: Average time to select a target (click/touch), in seconds, milliseconds, etc.
- $a$: Intercept. The **baseline response time** component determined by the device, user, and task.
- $b$: Slope. The **increase in time per additional bit of difficulty** (seconds/bit). Varies by device, user, and conditions.
- $D$: Distance from the starting point to the center of the target (or target area), in a consistent unit such as pixels or millimeters.
- $W$: Effective target width (tolerance) along the direction of movement or approach: the width within which a hit counts, in pixels, millimeters, etc.

## Articles

- https://www.smashingmagazine.com/2022/02/fitts-law-touch-era/