Thumb zone

Hold a phone in one hand and the thumb pivots from a fixed point near the bottom corner, sweeping an arc across the screen. Everything inside that arc can be tapped without moving the hand. Above it is a band that needs a stretch, and the far top corner, diagonally opposite the thumb, needs a regrip or a second hand. The zone is usually drawn as a heat map over the screen, warm at the bottom and cold at the top, and the useful part of the picture is that the arc has a fixed radius while phones keep getting taller: a 6.7 inch screen has the same reachable region a 4 inch screen did, and a great deal more that is out of it.

The vocabulary comes from field research on how phones are actually held, most of it traceable to Steven Hoober’s observation that a large share of phone use is one handed and thumb driven rather than the two handed, index finger posture interfaces were being drawn for. Two details matter more than the exact percentages. The map is handed, so a right thumb’s easy region is the bottom right and a left grip mirrors it, which makes the honest common ground a band across the bottom of the screen rather than one corner. And the arc is a comfort gradient, not a boundary: the stretch band is reachable, just slow and unstable, which is exactly the wrong quality for a control people hit dozens of times a session.

Most mobile conventions are this map made concrete. Bottom navigation sits where it does because the top of the screen is the worst place for the thing tapped most; the floating action button parks the primary action in the corner nearest the thumb; action sheets rise from the bottom edge instead of appearing as centred dialogs. Browsers moved their address bars down for the same reason, and both mobile platforms ship a system gesture that slides the whole screen into reach for the cases that cannot be designed around. The zone also has a reading in the other direction: putting a destructive or irreversible control in the hard band is a legitimate way to make it deliberate, as long as it is discoverable.

Two limits are worth stating. The thumb zone answers where a control goes, not how big it is, so it never substitutes for touch target size: a small target in the easy band is still a mis-tap. And the bottom edge is contested space, shared with the home indicator and the browser chrome, so a reach-friendly bar has to respect the safe area or it trades one problem for another. Grips also vary more than any single diagram admits, since people switch hands, use two thumbs while typing, and hold a phone differently when walking, which is why the map is a heuristic for placing the one or two actions that matter rather than a rule that governs a whole layout.