Fitts law

(In HCI) Fitts’ law.

T = a + b \log_{2} (1 + \frac{D}{W})

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.

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