Cross-currency rate calculator

What a cross-currency rate measures and how the triangulation works

If both quotes are expressed as units of that currency per one unit of the base, one unit of the source currency is worth one divided by from_rate units of the base, and that base amount buys to_rate divided by from_rate units of the destination currency. That quotient is the cross rate in destination units per one source unit. If both quotes are instead expressed as units of the base per one unit of the currency — the convention many dollar-per-euro and dollar-per-yen screens use — the identity flips: the cross rate is from_rate divided by to_rate.

Input contract, quote conventions, and response fields

Both rates and the amount must be strictly greater than zero: a zero or negative quote would make the implied pair undefined or reverse the economics of an exchange, and a non-positive amount is rejected for the same classroom domain. POST a JSON body with two required finite numbers: from_rate, the source-currency quote against the shared base, and to_rate, the destination-currency quote against that same base. Optionally send amount as the source quantity to convert (default one), quote as units_per_base or base_per_unit (default units_per_base), and decimals as an integer from zero through twelve (default six) to control half-up rounding on every numeric field. Missing fields, empty strings, non-finite values, an unknown quote token, and non-integer decimals outside the allowed range fail with invalid_input and are not billed. Common aliases are accepted: source_rate, rate_from, or from_vs_base for from_rate; dest_rate, target_rate, rate_to, or to_vs_base for to_rate; qty or quantity for amount; quote_mode or convention for quote. On success the body returns the echoed inputs, quote, cross_rate as destination units per one source unit, inverse_rate as the reciprocal, converted as amount times the cross rate, base_value as the source amount expressed in the base, and formula. Example: from_rate zero point nine two, to_rate one hundred fifty, amount two hundred, quote units_per_base yields a cross near one hundred sixty-three destination units per source unit because one hundred fifty divided by zero point nine two is the yen-per-euro identity when both legs are quoted per dollar. Pair the numbers with your own live feed if you need a timestamped mid; this endpoint computes only the implied pair from the two rates you already hold.

Where a cross-currency rate API earns its keep

Trip planners and multi-country itineraries use a cross-currency rate calculator so a budget written in euros can be restated in yen, pesos, or pounds from two quotes against the dollar without waiting for a dedicated pair that a boarding pass or hostel invoice never prints. Corporate travel desks and expense tools need a deterministic oracle that rejects a zero quote instead of inventing a hollow conversion a buggy sheet might treat as parity. Failed validation does not consume balance. Exam generators and open-economy problem sets ask for the triangular identity in both quote conventions; pinning the arithmetic in a tiny parse API keeps spreadsheet drift out of the mark scheme. Keep requests small, pin the rounded cross_rate and converted fields in tests, and you get reproducible implied pairs for homework, LMS plugins, and edge workers without standing up a live FX pipeline or confusing this identity with a purchasing-power real rate.