I believe it is standard in currency exchange rates to set a fixed precision for a given currency rate. The current state of the Frankfurter API makes i infeasible to obtain that information, however, due to the fact that trailing zeros are truncated.

When you look at the response from let's say the Polish national bank:
https://api.nbp.pl/api/exchangerates/tables/B?format=json

You can clearly see that the JSON numbers have trailing zeros, which is part of the spec. How to read that when parsing is another story, but it is still obtainable. It would be extremely helpful if we could have that in Frankfurter. Also the NBP table A is a good and as far as I checked free provider. The B table is updated weekly sadly, but together they provide all currency rates with PLN as the intermediary.