She asked for one thing
We were travelling, standing in a shop, adding up what was in our hands. A converter app gives you one number at a time. A calculator gives you a total in the wrong currency. Either way you do the sum twice, and by the end you have no idea what any single item cost in money you understand.
She wanted to see the whole sum in both currencies at once, while she was still typing it. I went looking for that app. Every calculator ignored exchange rates, and every converter had one input box and no arithmetic. There was nothing in between.
The whole sum, in both currencies
So the app shows two lines. 50 + 100 + 25 × 3 comes to 225 US dollars. Underneath, the same expression converted term by term — 7,331.79 + 14,663.58 + 3,665.90 × 3 — comes to 32,993.06 yen, with the rate it used sitting in the corner.
Every number is converted where it stands and colour-matched to the number above it, so you can read across the line and see which price was which. That is the whole app. Everything else exists to serve it.
One night, by hand
I built it in a single night, in 2023, and I typed every line of it — no AI in the loop. Flutter, a public exchange-rate file, a cache on the device, and the arithmetic to keep the two lines in step.
One night was enough because the app only ever had to do one thing. It still only does that one thing: no account, no sign-in, nothing to set up before it is useful.
What a trip actually needs
Every major currency and the cryptocurrencies sit in one searchable list, so changing the pair takes a second — and the second line works the same whether you are converting to yen or to ether.
Rates are cached on the device, so when the hotel Wi-Fi gives up the last rates it downloaded still work. It is free with ads, a one-off purchase removes them, and I never see what you convert.
