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Why my app doesn't convert currencies

· 3 min

I drive between Poland and Sweden. I fill up in kronor and get the car serviced in złoty. The first idea was obvious: convert everything into one currency and show a single total.

Three rates, three different totals

The rate on the day of the fill-up, today’s rate and the monthly average give three different results for the same receipts. None of them is “the real one”. A total in one currency would look certain while being only a guess.

What I did instead

Each currency keeps its own totals. A receipt for 412 kronor stays 412 kronor, and the statistics match what is in the glove box.

  • less magic, more trust in the numbers,
  • no silent assumptions about the rate,
  • simpler code.