Fitts's Law: make the important thing bigger and closer.

Sit in any car and look down. The brake pedal is bigger than the gas pedal, and it sits a little closer and lower, so your foot finds it fast. That is not a styling choice. It is Fitts’s Law, described by the psychologist Paul Fitts in 1954, and it is quietly deciding how your interface feels too.

The law is simple. The time to reach a target depends on how far it is and how big it is. Closer and bigger is faster and easier to hit. Farther and smaller is slower and more error prone. I have leaned on this one rule designing interfaces used by more than two million people, because it turns a vague sense of effortless into something you can place and measure. Here is how to use it.

Key takeaways

• The time to hit a target grows with distance and shrinks with size (Fitts, 1954). Important actions should be big and near.
• The average finger pad is 10 to 14mm wide (MIT Touch Lab), so make tap targets at least 44pt (Apple), 48dp (Material), or 44 CSS pixels (WCAG 2.5.5).
• Put the primary action where the hand already rests. On a phone, that is the thumb zone at the bottom, not a link in the top corner.
• Make rare and destructive actions smaller and set them apart, so a careless tap does not become a regret.
• A screen edge stops the cursor dead, so a target against it acts as infinitely large. A full-width bottom button is effortless.

What is Fitts’s Law?

It is a prediction, from 1954, about how long it takes to point at something. Paul Fitts found that the time to move to a target depends on two things: the distance to it and its size. Double the distance and it takes longer. Double the size and it takes less. Closer and bigger wins, every time.

The brake pedal is Fitts’s Law in steel. It is larger than the gas pedal and sits a little closer and lower, because the one moment you need it, you need it now. Your screens run on the same rule. The action you want people to take should be the easy target, and the layout either helps the hand or fights it.

How big should a tap target be?

Big enough for a finger, which is larger than most designers draw. The MIT Touch Lab measured the average finger pad at 10 to 14mm and the fingertip at 8 to 10mm. A target smaller than that asks people to aim, and aiming is where taps miss.

The platforms agree and gave you numbers. Apple’s Human Interface Guidelines call for at least 44 by 44 points. Material Design says 48 by 48 density-independent pixels with 8 of spacing. The accessibility standard WCAG 2.5.5 sets 44 by 44 CSS pixels. When in doubt, 44 is the floor, not the goal. Go bigger for the action that matters most.

Where should the primary action go?

Where the hand already rests. On a phone held in one hand, that is the thumb zone across the bottom of the screen, not a small link in the top corner that asks for a stretch and a squint. Put your main call to action where the thumb falls, and it stops feeling like a reach.

This is why bottom bars, full-width buttons, and bottom sheets took over mobile design. They are not a trend. They are Fitts’s Law applied to a thumb. The closer the target sits to the resting hand and the larger it is, the more the interface feels like it is meeting the user halfway.

What about dangerous actions?

Make them harder to hit, on purpose. The rule cuts both ways, and the second half is where good design shows. Delete, pay, confirm forever. Keep these smaller, set them apart, and give them a moment of distance, so a careless tap does not turn into a regret you have to undo.

Friction is usually the enemy, but here it earns its place. A destructive button that is a little smaller and a little farther, or guarded by a second step, costs a fraction of a second and saves the occasional disaster. Use size and distance to say what matters, and to whisper what should give a person pause.

Why do edges and corners feel so good?

Because a screen edge stops your cursor or thumb dead. You cannot overshoot it. In Fitts’s terms a target against an edge behaves as if it were infinitely large in that direction, so it is effortless to hit even without looking. This is the trick behind a button that fills the bottom of the screen, and behind the menu that sits hard in the corner of a desktop.

Use the edges you have. A primary action pinned to the bottom edge of a phone, running the full width, is one of the easiest targets you can make. The frame of the screen is doing half the work for you.

The two-minute audit

None of this needs a redesign. Open your product and find the primary action. Ask if it is the biggest, nearest, easiest thing to hit. Then find your most destructive action and make sure it is not. If the answers are backwards, you have your afternoon.

Size and distance are not decoration. They are how an interface tells the hand what matters. Get them right and the product feels effortless without anyone being able to say why. If you want that applied to something you are building, tell me what it is.

FAQ

What is Fitts’s Law?

Fitts’s Law states that the time to move to a target depends on the distance to it and its size. Closer and larger targets are faster and easier to hit, which is why important actions should be big and within easy reach.

How do I apply Fitts’s Law to UI design?

Make the primary action large and place it where the hand already is, such as the thumb zone at the bottom of a phone screen. Make rare or destructive actions smaller and set them apart so they are harder to hit by accident.

Why are screen edges good for buttons?

An edge stops the cursor or thumb, so a target against it acts as if it were infinitely large in that direction. A button that fills the bottom edge of the screen is effortless to hit, which is Fitts’s Law working in your favor.