Fitts's Law Formula
Predicts how long it takes to reach a target from its distance and size.
T = a + b × log₂(2D / W)
Worked example: a = 100 ms, b = 150 ms, D = 120 px and W = 60 px. T = 100 + 150 × log₂(4) = 400 ms.
Why it matters
It lets you compare two layouts with numbers, without a test.
How to use it
- Measure D as the distance to the target.
- Measure W as its size along the direction of movement.
- Get a and b from real users where you can.
Example
Doubling the width reduces the difficulty by one bit.
Watch out
Researchers now often use the Shannon form, log₂(D / W + 1). The idea is the same.