Guide · Invoicing

Which exchange rate to put on an invoice

You invoice a customer in dollars, your books are in euros, and the amount has to land in both currencies. The exchange rate you pick is not a detail: it decides the euro figure on the invoice, the VAT you owe, and whether the number survives a question from an auditor or a customer’s finance department. This guide walks through the choice most small companies actually face.

The short version: use one official reference rate for the date the invoice applies to, write down where it came from and when it was published, and use the same rule every time. The rate itself matters less than the rule — an auditor who can replay your calculation does not argue with you.

The candidates, and what each one is

Four rates tend to compete for the spot on an invoice. They are different numbers, and knowing which is which is most of the answer.

The central bank reference rate

A rate published by a central bank or official source, for example the European Central Bank’s euro foreign exchange reference rates, published once per working day at around 16:00 CET. It applies to a specific date, is reproducible by anyone, and is what tax authorities in many countries expect to see quoted.

The rate on the payment date

Useful when the invoice is settled weeks after it is issued and the actual bank settlement differs from the invoice figure. The difference is an exchange gain or loss, not a reason to rewrite the invoice.

The rate your bank or payment provider applied

This is what your account actually received, fees included. It is a reconciliation fact, not a pricing decision — putting it on the invoice means every customer sees a different, unexplained rate.

A blended or averaged rate

Some APIs return an average of several market sources with no stated origin. It looks reasonable and cannot be reproduced: when someone asks where the number came from, there is no answer.

A worked example

Say you invoice a US customer 10,000 USD on 2026-10-03 and your books are in euros. The latest ECB euro foreign exchange reference rate at that point was published on 2026-10-02: 1 EUR = 1.1225 USD. The euro figure on the invoice is:

Those four fields — the rate, the institution, the publication timestamp, the date it applies to — are the whole defence. If the customer questions the amount next quarter, you replay it in one line. Note the one honest wrinkle: the invoice is dated 2026-10-03 and the rate applies to 2026-10-02, because the ECB had not yet published on the invoice date. That is normal for official daily rates; the rule is simply “the latest official rate available on the invoice date”, stated and kept.

Three rules that hold up

1. One source, always. Pick one official source for each currency pair and never mix. Mixing sources is what makes two invoices for the same amount disagree.
2. Record the date and the origin, not just the number. A rate without its source and date is a rumour on your invoice. Store them with the invoice record, in the same row.
3. Never round the rate to make the number prettier. A reference rate of 1.1225 is 1.1225. If you display fewer digits, say so; do not silently recompute.

One caveat we will not skip: this is general guidance, not tax advice. Which date counts for VAT, and which sources your tax authority accepts, depends on your country. Ask your accountant which rule applies to you — then apply it with the discipline above.

Getting the rate, with the fields attached

GlobalCurrencyLayer returns central bank reference rates with the source, publication timestamp and valid-for date attached to every number, so the record you keep is the record the API gave you. The free plan covers small volumes; the details are on the pricing section of our home page.

GlobalCurrencyLayer itself is built and operated end to end by AI agents on NanoCorp, which is how a two-person company publishes an API at all.