interaction design
Thumb zone
What is the thumb zone, and why does it matter on mobile?
The thumb zone is the area of a phone screen a person can comfortably reach with the thumb of the hand that's holding it - roughly the lower and centre of the display. The far corners, especially the top, are a stretch or need a second hand. Put the things people tap most inside the easy zone, and keep the awkward corners for the rare and the harmless.
Also known as: thumb zone, thumb-friendly, reachability, thumb reach
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The demo
A phone, held one-handed. Move the primary button between the thumb zone and the top corner, and feel the difference in reach for something you'd tap all day.
What this demo shows (text version)
A phone frame is shaded to show reachability for a one-handed grip: the lower and central area is the easy "thumb zone", while the top - especially the opposite top corner - is hard to reach. A toggle places the primary "Checkout" button either across the bottom (in the thumb zone) or stranded in the top-right corner.
In the thumb zone the button sits where the thumb naturally rests, so a frequent action is one easy tap. In the top corner the same button needs a stretch or a second hand. Because reachable area shrinks as screens grow, primary and frequent actions belong low; reserve the awkward corners for rare or deliberately cautious actions. It's Fitts's law applied to how phones are actually held.
On a one-handed phone, the bottom and centre of the screen are easy for the thumb and the top corners are a stretch - so primary, frequent actions belong low, not marooned in a top corner. It's Fitts's law on a moving target: as screens grow, "reachable" shrinks relative to the whole, which is why bottom bars and bottom sheets keep winning on mobile.
In the thumb zone the button met your thumb where it already rested - one easy tap. Stranded top-right, the same button needed a stretch or a second hand for an action you'll do constantly. Nothing about the button changed; only whether it sat where the thumb can actually go.
Reachability is Fitts's law with a twist: the target isn't just small or far, it's far in a way that depends on how the device is held. One-handed, the thumb sweeps an arc from the bottom corner; the opposite top corner is the worst spot on the screen. Frequent, primary actions belong in that comfortable arc - which is why bottom navigation, bottom sheets and reachability gestures exist.
Bigger screens make it worse, not better. As phones grow, the share of the display a thumb can reach shrinks, so a "top bar" that was fine on a small phone strands its controls out of reach on a large one. Designing for the thumb zone is increasingly designing for the bottom of the screen.
It's a guideline, not a straitjacket. The top corners aren't forbidden - they're the right home for the occasional and the cautious: a rarely-used settings cog, or a destructive action you'd rather people didn't hit by accident. Match position to frequency and stakes, and remember people hold phones differently, so don't bet everything on one grip.