Built for one hand, because the other one is holding something

The single most common criticism of currency apps in their own reviews is not accuracy. It is that the keys are too small to hit.

Picture where you actually convert currency. Standing at a market stall with a bag in one hand. On a train, holding a rail. At a counter with a queue behind you. Outdoors, in daylight, with a screen you can barely see.

Now picture the interface most converter apps give you: a dense list, a small text field, and the standard iOS keyboard — which puts the number row across the top of the screen, exactly where a thumb cannot reach.

What the reviews actually say

Read one- and two-star reviews of popular converters and a specific complaint repeats: the keypad. People describe keys they cannot read in sunlight, entry fields where the number they are typing is hidden behind something, and layouts that need two hands and full attention for what should be a two-second task.

These are not accuracy complaints. Every converter gets the arithmetic right. They are complaints about being asked to stop what you are doing and concentrate.

What we changed

• A custom number pad, not the system keyboard. Large keys sized and positioned for a thumb, in the lower part of the screen where a thumb actually reaches.
• Haptic feedback on every key. You can feel that a digit registered without looking, which matters when you are half-watching the person you are negotiating with.
• High contrast by default. The app is black with white figures, which stays readable in direct sun rather than becoming a mirror.
• The result is always on screen. No submit step, no separate results view. Type and the converted figure is already there.
• Nothing in front of the keypad. No ad, no interstitial, no rating prompt at the moment you opened the app to do one thing.

Why this is the feature we would lead with

Working offline is table stakes — Apple ships an offline converter for free and most paid apps claim it too. Being usable with one hand, on your feet, without concentrating, is the part almost nobody builds for, and it is the difference between an app you keep and an app you delete after one trip.