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.

Worked example: an invoice dispute over a 2018 contract

A US company invoiced a UK client £24,000 on 15 March 2018 for services rendered. The contract specified payment in USD “at the prevailing exchange rate on the invoice date.” The client paid $32,000 in May. The supplier believes they were short-paid. What does the data say?

1. Identify the right source. For GBP/USD on a 2018 working day, both ECB and the Federal Reserve H.10 series cover the date. ECB’s reference rate is published once per business day around 14:15 CET; the Fed’s H.10 fixes at noon Eastern. Either is defensible — but ECB is the European convention and the contract was for a UK client, so ECB is the cleaner citation.
2. Look up the rate. ECB’s 15 March 2018 reference rate was GBP/USD = 1.3979 (1 GBP buys $1.3979).
3. Compute the contractual amount: £24,000 × 1.3979 = $33,549.60.
4. The supplier was short-paid by $33,549.60 − $32,000 = $1,549.60, roughly 4.6%. The 4.6% gap is consistent with a typical bank-wire spread (2-3% mark-down each direction). The client almost certainly used their own bank rate rather than mid-market, breaching the contract’s “prevailing exchange rate” language if interpreted as mid-market.

For court purposes, cite the ECB statistical-data-warehouse URL with the specific time-series identifier rather than a screenshot. The identifier for GBP/USD is EXR.D.USD.GBP.SP00.A; the URL pattern is stable and the rate is reproducible by anyone with a browser.

Common mistakes when sourcing historical rates

• Confusing “published on” with “effective for” date. ECB publishes the rate at 14:15 CET that captures market data from14:00 CET. For a trade executed at 11:00 CET, that day’s ECB rate is the closest available reference, but it’s not the exact rate at trade time. For court-grade evidence on intraday trades, you need Refinitiv or Bloomberg tick-data, not the daily reference series.
• Treating IRS yearly-average rates as spot-equivalent.The IRS publishes a single annual average for tax-reporting convenience; the actual rate on any specific date in that year can differ by 10%+. For contractual disputes use the specific date’s rate. For US tax returns where the IRS explicitly allows the annual average, use that.
• Mixing buy and sell rates.Travel-money counters publish two rates (the rate they buy your currency at, and the rate they sell at). The spread is often 5-8%. Historical archives of these rates aren’t comparable across sources without normalising to the mid-market.
• Forgetting redenominations.The Turkish lira was redenominated 1:1,000,000 on 1 January 2005 (1 new lira = 1 million old lira). Many historical tools mis-handle the discontinuity. Same problem for the Romanian leu (2005), the Belarusian rouble (2016), and several others. Always check the issuing central bank’s redenomination notes for any currency you look up pre-cutoff.
• Using an LLM as the source.Modern chat models hallucinate historical FX rates frequently and confidently. Always cite a primary archive; verify even “simple” lookups against ECB or the relevant central bank.

When historical-rate lookup does NOT apply

• Hyperinflation regimes.Zimbabwe 2007-09, Venezuela 2016-present, Weimar Germany 1923. A “daily mid-market rate” is essentially meaningless when the rate halves every few weeks. Reconstruct purchasing-power equivalents via a basket of goods rather than nominal FX.
• Pre-fiat / commodity-money periods. Before 1971 (Bretton Woods exit), most currencies were pegged to gold or to USD. The published “rate” was the official peg, not a market price; black-market rates often diverged 20-50%. For long-historical analysis use MeasuringWorth or academic series, not present-day FX tooling.
• Sanctioned or non-deliverable currencies. Iranian rial, North Korean won, Cuban peso historically have multiple parallel rates (official, market, NDF). No single source captures all of them; document the rate type alongside the value.

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.

Citation format for legal and accounting use

For court, audit, or tax filings, a defensible citation includes four elements:

1. Source organisation (European Central Bank / Federal Reserve / IMF)
2. Specific dataset identifier (e.g. ECB time-series code EXR.D.USD.GBP.SP00.A)
3. Reference date in ISO 8601 (2018-03-15, not “3/15/18”)
4. Retrieval date — the date you pulled the data, in case the underlying series is later revised

Sample citation: “European Central Bank, Euro foreign exchange reference rates, GBP/USD series EXR.D.USD.GBP.SP00.A, rate on 2018-03-15 = 1.3979, retrieved 2026-05-21.” This format survives challenge from opposing counsel because it points anyone with a browser to the exact reproducible value. Screenshots and convenience-site URLs do not.

Source coverage matrix by date and currency

Which source has what. Use this to pick the right archive for your specific date and pair:

For a typical accounting query on a major pair in the last 20 years, ECB (EUR-quoted), Fed H.10 (USD-quoted), or BoE (GBP-quoted) covers it free of charge. For anything pre-1971, the IMF and MeasuringWorth split the academic territory between them; expect annual or monthly granularity at best.