Fitts's Law

Time to acquire a target depends on its distance and size. Bigger + closer = faster.

How to apply it

Place primary actions in the bottom third and make them ≥44×44pt.

Fitts's Law in practice: a concrete example

Moving the checkout button into the thumb zone

On a 6.7-inch phone, a “Pay” button stranded in the top-right corner forces an awkward reach and invites mis-taps. Move it to the bottom of the screen and make it full-width. Now it's a big, close target: faster to hit and far harder to miss. This one change routinely lifts mobile checkout completion.

Common Fitts's Law mistakes

Tap targets under about 44px, or placing them so close together that Delete sits a pixel from Confirm.
Wasting the “infinite edges”. Screen corners and edges act like enormous targets because you can't overshoot them, yet primary actions rarely use them.
Putting a destructive action right next to the primary one, where a slightly-off tap does real damage.

When Fitts's Law doesn't apply

Flip it on purpose for dangerous, irreversible actions. A “Delete account” button should be smaller and farther away, not the biggest, closest thing on screen. Here friction is a feature: the extra distance is what prevents mistakes.

Practise it on a real challenge

Notes App — the button no thumb can reach

A notes app took off on 6.7-inch phones, but 'New note' lives top-right — the one corner a thumb can't reach one-handed. Reviews are piling up about how awkward it is one-handed.

Match each UX law to the problem it solves

Usability bugs are easy to spot but hard to name. The right law points straight to the fix.