A converter is easy to write and easy to get quietly wrong. The failure is rarely dramatic: a rounding applied one step too early, a definition that changed in 1959, a locale that reads 1,5 as fifteen. Nobody notices until someone prices a shipment.
So each tool ships with a reference: the defining document for the unit or the formula, linked at the bottom of the page. Weight and length conversions cite NIST and BIPM. Finance tools cite the CFPB and Freddie Mac, and their arithmetic is unit-tested against the published formulas — the amortisation maths against the standard annuity formula, the tax bands against the figures the IRS prints.
Three checks, every tool
First, the boundaries: zero, one, negative, absurdly large, and the value where a formula divides by zero. Second, the locale: comma decimals, spaces as thousand separators, and the German and Dutch labels at their longest. Third, the phone: every tool has to be usable and readable at 375 pixels wide before it counts as finished.
The mortgage calculator took the longest, not because the formula is hard but because the amortisation schedule is 360 rows and a phone is 375 pixels wide. It ended up as twelve records you can expand rather than a table you pinch and drag.