Fitts' Law
The time to acquire a target is a function of the distance to and size of the target.
Plain English
Fitts' Law says: bigger and closer = easier to click. A large button close to the cursor takes less time to hit than a small button far away. This is why: Mac's menu bar sticks to the screen edge (infinite height — you cannot overshoot it), why CTAs should be large and centrally placed, and why mobile touch targets must be at least 44×44px. The law also explains why corners and edges of a screen are the easiest targets — you cannot scroll past them.
Research background
ID = log₂(2A/W), where ID is the index of difficulty, A is the distance from start to target, and W is the width of the target. The model predicts movement time as: MT = a + b × ID. Screen edges and corners are special — they have effectively infinite size in one dimension, making them zero-difficulty in that axis. Applied to UI: larger targets reduce MT; proximity to the starting point reduces MT.
UI examples
✓ Applying the principle
Making the primary CTA button at least 44px tall and placing it where the user's cursor naturally lands after reading — dramatically reduces time-to-click.
✗ Violating it
A 12px × 12px ×close button in the corner of a small modal — small target, far from the typical cursor position, high miss rate.
AI Prompt
Paste into Claude, Cursor, Bolt, or v0.
Apply Fitts' Law: minimum 44px touch targets on all interactive elements, larger for primary CTAs (52-56px height), and place the most important action where the eye naturally lands after reading the content.