Where should the primary button sit on a mobile screen?
In the bottom third of the screen, inside the thumb's natural arc. About half of people use a phone one-handed, and the thumb sweeps a curve from the bottom corner. Distance is effort, and effort decides taps: that is Fitts's law working. Put the primary action where the thumb rests. Put destructive actions where it has to reach.
Pick up your phone. Watch your own thumb. It rests near the bottom, sweeping a small arc.
That arc is the most valuable real estate in your product.
The reach map. Steven Hoober's research logged 1,333 real-world observations of people using phones. 49% of those touching the screen held the phone one-handed. The thumb reaches the bottom third easily, the middle with a stretch, the top corners with a regrip.
Every regrip is a cost. Costs get paid only when motivation is high. A primary button should never require one.
Fitts's law, in one line. The farther and smaller a target, the longer and harder the tap. Close and big wins. It is physics, not taste.
The bottom is the default zone. A button under the thumb becomes the path of least resistance. Defaults win decisions. Placement is a default.
This is why tab bars live at the bottom and why checkout buttons stick to the screen edge. The interface meets the thumb where it already is.
Use distance on purpose. Reach is a persuasion tool with two directions. Easy reach for the action you want. Deliberate distance for delete, cancel, and anything irreversible. Friction is protection when you place it deliberately.
Top corners are for actions used once a session or less. Settings, filters, back.
My own site does this: on a phone, the booking action sits fixed at the bottom. Not decoration. Reach.
The easiest tap wins. Choose which tap is easiest.
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