APR & APY Calculator

APR & APY Calculator converts between APR (Annual Percentage Rate) and APY (Annual Percentage Yield), and compares interest rates across different compounding periods — no spreadsheet required.

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Origin story

I built this app while I was into crypto: trading, exchanges, transfers, DeFi, staking.

I was doing some pretty risky decentralized coin staking — at a minimum of 10,000% (ten thousand percent) APY. With numbers like that you can't help but think of Warren Buffett and his compound-interest chart: at 3–10% a year, compounding pays off, but only over decades. Here, interest could accrue every single second — and it got interesting to work out when it was optimal to withdraw it and roll it back into the principal, so it would start earning interest too.

I went looking for calculators, but ones that compute interest for periods shorter than a month barely exist (and fair enough, who'd need that? :]) — I needed to calculate compound interest down to the second.

So I got to work ("Ah shit, here we go again"). I already knew how to make games in Unity, and figured the fastest way was to build the app right on the game engine — overkill as that was.

But building charts in Unity was its own kind of torture: back then all it had was Unity UI, with none of the convenience CSS gives you, and the load of running the compound-interest formula — even for a single month, re-adding interest every second — would freeze any phone!

So the app ended up with two stages in its life: a Unity release, and then — a few months later, once it was clear this couldn't go on — a rebuild in Flutter.