Fitts' Law

Fitts' Law describes how quickly people can hit a target. This applies to any targeted movement: the finger to the button, the mouse pointer to the close cross, the thumb to the key on the phone. The rule behind it is simple. Two things determine the duration, namely the distance to the target and the size of the target. Far away and small means slow, near and large means fast. The US psychologist Paul Fitts measured this relationship in experiments in 1954 and put it into a formula.

Why designers argue about button sizes

Fitts' Law is one of the few rules in interface design that can actually be calculated. Much of design is a matter of taste. Hit time is not. That’s why the law appears in almost every usability textbook.

In practice, this means: Important buttons should be large and close to the place where the pointer already tends to be. Rarely used or dangerous functions can be placed smaller and farther away. The delete button, after all, should not be reachable with a mere twitch. So the law explains not only speed, but also errors.

A second point concerns money. If a buy button on the phone is too small, users abandon the process more often. Large online retailers test such details systematically. A few more pixels of target area can mean measurably more completed orders.

Distance, target size, and the rest of the formula

The formula relates movement time to the ratio of distance and target width. Doubling the distance doesn’t cost twice as much time, just somewhat more. Halving the target size also only costs somewhat more time. The increase is thus dampened, not linear.

The reason lies in the way we aim. First comes a quick, rough swing in the right direction. Then follow small corrections until the pointer sits in place. The swing is fast, the corrections eat up the time. With a large target, most of the corrections simply drop away.

An often overlooked trick concerns screen edges. The mouse pointer cannot go beyond the edge. A menu right at the top edge is therefore practically infinitely tall, because you can just yank the mouse upward. This is exactly what the menu bar at the top edge on macOS thrives on. Corners are even better, because two edges meet there.

From the Start menu to the touchscreen

On the phone, the law shows itself every day. Apple and Google recommend a minimum size of about seven to nine millimeters edge length for tap targets. Smaller becomes unreliable, because the fingertip is an area and not a point. Anyone who has ever hit the wrong link on a mobile website knows the result.

Classic software also provides examples. The right-click menu appears directly under the pointer, so the distance is nearly zero. Pie menus in graphics programs use the same principle. Conversely, the Start button of earlier Windows versions is considered a textbook case of a squandered advantage, because it did not sit exactly in the screen corner.

In the world of AI, Fitts' Law takes on new meaning. Assistants suggest answers that you accept with a single tap. The closer and larger this suggestion is, the more likely it is to be accepted. Critics therefore speak of dark patterns, meaning design that deliberately nudges in one direction. A consent button large and centered, the decline option small at the edge — that is Fitts' Law, used against the user.