Building a Cross-Country Payroll API: The Weird Problems I Didn't Expect

webdev #fintech #ai #javascript

Last year, while trying to estimate the real take-home pay of a potential job abroad, I started building my own German payroll tax engine.

At first it was a simple question:

"What would actually be left from the salary after taxes and deductions?"

But once the German calculation layer existed, adding other countries felt almost inevitable. So I expanded into Austria, Switzerland, the UK, Ireland, the US, Canada, and Australia — the goal being a unified API layer for cross-country salary comparison.

The problem was that the deeper I went, the less universal the concept of "salary" actually became.

The "same salary" illusion

My initial assumption was that cross-country salary comparison was mostly a math problem: feed in a gross salary, apply taxes, compare the outputs.…