Fitts's Law
The farther away and the smaller a target is, the longer it takes to reach.
T = a + b × log₂(2D / W)
Worked example: With a = 100, b = 150, D = 120 and W = 60, the time is 100 + 150 × 2 = 400 ms.
Why it matters
Every tap or click is a small physical task. Hard targets slow people down and cause mistakes.
How to use it
- Make frequent and important actions larger.
- Place main actions close to where the thumb or cursor already is.
- Leave space between actions that could be confused.
Example
A full-width Pay button at the bottom of a phone screen is far easier to hit than a small icon in the top corner.
Watch out
Bigger is not always better. Keep destructive actions like Delete harder to hit by accident.