How to convert currency for a historical date (and why most online tools are wrong)

Convertitive’s currency converters show today’s mid-market rate. For invoices, accounting, legal disputes, or curious-historical lookups, you need the rate on aspecific past date — which is a different problem with different sources.

Why our converter doesn’t do historical rates (yet)

Historical FX data is much more storage- and bandwidth-heavy than spot rates. 50 currencies × ~14,000 business days since 1999 × N decimals = hundreds of megabytes of time-series data. We’d need to host that, refresh it daily, and rebuild the cache when corrections come in. The feature is on our roadmap; today it isn’t built.

Until then, here’s where to get the data yourself.

The four reliable sources

1. ECB Statistical Data Warehouse (free, 1999-present)

The European Central Bank’s SDW exposes every daily reference rate the bank has ever published — going back to the euro’s introduction in January 1999. Free, no signup, downloadable as CSV.

Direct URL: https://data.ecb.europa.eu/data/datasets. Choose the “Exchange rates” dataset, pick your currency pair, set the date range.

ECB rates are the canonical mid-market reference rate for the eurozone. For any conversion involving EUR, GBP, USD, JPY, or any other major currency since 1999, ECB is the authoritative answer.

2. OANDA (free for casual use, full archive back to 1990)

OANDA’s historical converter at oanda.com/currency-converter covers ~200 currencies back to 1990. Free for a few lookups a day; a paid plan unlocks bulk export.

OANDA rates are interbank averages from their own aggregation, not ECB’s feed — they’ll agree to about 0.5% for major pairs. For accounting purposes either is defensible; for legal proceedings, ECB is typically the preferred citation.

3. IMF (free, monthly + annual averages, 1948-present)

The International Monetary Fund publishes monthly and annual exchange-rate averages back to 1948. Free, well- documented, used by every academic paper that needs long- term FX time series.

Limitation: daily rates aren’t available. For anything pre-1990 where you only need monthly granularity, IMF is the right source.

4. National central banks (varies, free)

For non-major currencies, the issuing country’s central bank usually publishes a historical archive of their currency’s reference rate against USD or EUR. Turkish lira: TCMB. Brazilian real: Banco Central do Brasil. South African rand: SARB. Etc.

Which rate to use

Three kinds of rate get published. For most use cases:

• Mid-market (reference) rate — the standard for accounting, legal, contracts. Use this unless told otherwise.
• Close-of-business rate — for end-of-day reporting. Often identical to the mid-market rate.
• WM/Refinitiv 4 PM fix — institutional benchmark used by funds to value cross-currency positions. Free of charge from Refinitiv with delay.

The day-of-week problem

ECB and most central banks don’t publish weekend or holiday rates because the FX market is closed. If your target date is a Saturday, Sunday, or local holiday, use the most recent published rate before that date. Some jurisdictions (notably the EU for VAT purposes) prescribe the previousbusiness day’s rate; others prescribe the next. Check your local accounting rules.

Why online historical converters disagree

Most free online historical converters source from one of the above plus add a spread or aggregation. They’ll disagree by 0.5-2% on the same date for the same pair. For anything that has to be defensible, cite the primary source directly (ECB / OANDA / IMF), not the convenience site that re-served their data.

What to expect from Convertitive

Historical rate lookup is on the roadmap — see currency methodology. Expected delivery: ECB daily archive for the 50 currency pairs we already support, going back to 1999, with the same ISR-cached page-per-pair pattern.