Designing for One-Thumb Mobile Navigation
On a food delivery app redesign, our session recordings showed something that should have been obvious but wasn't in our original design assumptions: the vast majority of users held the phone in one hand and navigated with their thumb, and the app's primary navigation, cart, filters, account, was a row of icons at the very top of the screen, the single hardest zone to reach one-handed on a modern large-format phone.
We measured actual thumb reach zones for a range of common device sizes and found the comfortable zone sits roughly in the bottom third of the screen, with the top third requiring a hand shift or a second hand entirely. Primary navigation moved to a bottom tab bar, and the few actions that had to stay at the top, like a back button, we kept but stopped treating as equally important to reach quickly.
The harder problem was the app's filter and sort controls, which had lived in a top sheet that slid down from the header. We redesigned it as a bottom sheet instead, which sounds like a small change but meant the entire interaction, opening filters, adjusting them, applying them, happened within comfortable thumb range without the user's grip shifting at all during the task.
We didn't move everything to the bottom reflexively, though. Destructive or rarely used actions, like deleting an account or clearing all filters, we deliberately kept slightly harder to reach, in the top corner or behind a secondary menu, because in that specific case friction is protective rather than annoying. One-thumb optimization is about matching reach to frequency and stakes, not minimizing every reach uniformly.
After the redesign, time-to-checkout in the delivery app dropped measurably, and support feedback mentioning the app being 'hard to use one-handed while walking,' a real and common context for that product, dropped to near zero. Designing for how phones are actually held, not how they look in a static mockup, turned out to be one of the highest-leverage changes we made on that project.
