Textbook of Usability

Fitts's Law predicts the time required to move to a target as a function of the distance to and size of the target. Formally:

$$T = a + b \log_2 \left( \frac{D}{W} + 1 \right)$$

where $T$ is movement time, $D$ is the distance to the target, $W$ is the width of the target, and $a$ and $b$ are empirical constants.

The implication for design is straightforward: bigger targets are faster to hit, and closer targets are faster to hit. The relationship is logarithmic, so doubling target size doesn't halve movement time — it produces a smaller improvement.

Fitts's Law has wide-ranging design implications:

• Buttons should be large when they will be clicked frequently
• Edges and corners of the screen are infinitely large in one dimension, so they're optimal for placing critical controls (the macOS menu bar lives at the top edge for this reason)
• Pop-up menus that appear at the cursor are faster than menus at fixed locations
• Touchscreens have minimum target sizes (typically 44pt or 48dp) to account for fingertip imprecision

The law was originally formulated by Paul Fitts in 1954 for hand movements, but it generalises to mouse, stylus, and touch input.