The thumb zone is reachable arc times visual attention, not the reachable area itself
Aliases: reachable arc · natural thumb zone · reach is not heat
What it is
A Buy now in the lower-right corner is green on many reach heatmaps: the thumb can get there. At the press, the pad covers the price beside it, and the eyes are still on the product image above — reachable, yet the confirmation is invisible. The thumb zone is the intersection of the reachable arc and visual attention, not the union of reachable pixels.
A map that only paints “can the thumb arrive” will recommend the dead corner of the bottom edge, the inner strip under the palm, and locations that can be reached only by tearing gaze off the content. Those cells are reachable. They are not hot.
Why it happens
In a one-handed grip the thumb travels cheaply inside an arc. Inside that arc three kinds of cell stay cold: covered by the pad, sitting under the heel of the palm, and sitting far from whatever is being looked at. A hot cell has to be cheap to hit and checkable with a short glance. The inward lower third, slightly above the physical bottom, often wins both; a dead corner flush with the bezel wins only reach.
Attention also tows the zone toward the content’s centre of gravity. During long-form reading, gaze sits mid-upper; a tab-bar icon that is reachable still needs gaze pulled down to be aimed, and heat drops. During an image task, gaze is central and the zone sits higher and more inward than a pure reach map.
Studying it
Sample two datasets on the same grid: reach (one-handed arrival time, effort ratings, grip shifts) and attention (whether a fixation landed on the cell before the press, whether the pad occluded it). Define the zone as cells that clear both lines, not cells that clear reach alone.
Independent variables: cell location, task type (reading / looking / form filling), handedness. Dependent variables: arrival time, proportion of pre-press fixations on the cell, proportion of adjacent information covered, subjective “I knew what I was pressing”.
Use real task content. An empty-button reach test erases the attention factor and the zone collapses back into reachable area.
Where it stops holding
Two-handed use, or the phone on a table tapped with the other hand, expands the zone toward the whole screen and the intersection constraint lifts for a while. A left-handed grip mirrors the intersection; a right-hand map must not be pasted on. On a tablet or an unfolded foldable the thumb arc covers a smaller fraction of the screen; painting the button larger does not restore the zone. Users whose visual attention is replaced by speech (walking with a screen reader) are not in this intersection; placement then follows reach and audibility.
Applying it
- Do not drop a third-party reach heatmap onto button slots. Overlay, on your own primary task, “can it be seen before the press, and does it cover a number that must be read”.
- Keep primary actions out of the physical bottom corner and the inner palm strip; sit them inward in the lower half, slightly above the home indicator.
- If a number must be read before the press (price, quantity), park it outside the pad’s projection and still inside one short glance of the control.
- How to check: film a one-handed run of the main task over the shoulder, freeze the press frame, and ask two questions — did the thumb stretch, and is the critical number still visible beside the pad. Both must pass before the slot is a thumb zone rather than merely reachable.