What is Fitts's law?

Fitts's Law describes the time required to move a pointer (e.g., a mouse cursor) to a target, depending on the distance to the target and its size. The closer and larger the target, the faster a user can reach it.

Law's formula:

[ T = a + b \times \log_2\left(\frac{D}{S} + 1\right) ]

where:

• T — the movement time to the target,
• D — the distance to the target,
• S — the size of the target,
• a and b — empirical constants.

In UX/UI, this helps optimize the placement of interface elements: large and important buttons are made bigger and placed closer to the expected click area to speed up user interaction.