Tax Authority Exchange Rate API

Official tax authority exchange rates via one API: UK HMRC's monthly customs and VAT rates and the US Treasury's quarterly reporting rates, as JSON, with history back to 2001. The fixed rates customs declarations, VAT invoices, and federal reports are required to use.

Why a tax authority rate is not a market rate

On any given day the market rate and the rate you are legally required to apply are two different numbers — and for customs and tax purposes only the second one counts. A tax authority fixes one rate for a period, publishes it, and expects every declaration in that period to use it, no matter where the market goes in between. Convert an invoice at the day's market rate instead and the duty or VAT figure will not reconcile with what the authority computes.

That is a feature, not a limitation. A rate fixed for a month or a quarter is what lets an importer, their broker, and the customs authority arrive at the same number independently — so a discrepancy means a real error rather than two systems that sampled the market hours apart.

How the cadences differ

The two sources solve the same problem on different schedules, and the difference matters when you integrate them:

• HMRC — monthly, published in advance. One table per calendar month, released around two weeks before the month it applies to, so next month's rate is knowable ahead of time. Any date inside a month resolves to that month's rate.
• US Treasury — quarterly, amended mid-quarter. One table per quarter dated at quarter end, with amendments issued for individual currencies that move sharply. An amendment covers only those currencies — so the newest publication date is often not a full table.

The API returns the table in force rather than the rows sharing the newest date: the period's rates with any later amendments already applied, each row carrying the date it was actually published. You get the state a filer would be reporting on today without reconstructing it from the raw feed.

Covered tax authorities

Live coverage — this table is generated from the same database the API serves, so it always matches /api/v1/central-banks:

Authority	Country	Code	Cadence	Currencies	History from	Latest table
HM Revenue & Customs	United Kingdom	hmrc	Monthly	150	January 2021	2026-08-01
U.S. Department of the Treasury	United States	ustreasury	Quarterly	187	March 2001	2026-07-15

More authorities are on the roadmap — Ireland's Revenue, Canada, Australia's ATO, and the EU's customs rates among them. Need a specific one? Tell us — coverage requests move sources up the queue.

Quick start

Same API keys, authentication, and endpoints as the rest of AllRatesToday — tax authorities are served by the same /central-bank/ routes. Ask for a pair with the familiar source/target parameters:

curl "https://allratestoday.com/api/v1/central-bank/hmrc/latest?source=GBP&target=USD" \
  -H "Authorization: Bearer art_live_your_key"

The response carries the publication date of the table the rate came from:

{
  "bank": "hmrc",
  "name": "HM Revenue & Customs",
  "rate_date": "2026-08-01",
  "source": "GBP",
  "target": "USD",
  "rate": 1.3367,
  "rate_type": "monthly",
  "derived": false,
  "method": "published"
}

Or the authority’s full published table — every currency it prints for the current period:

curl "https://allratestoday.com/api/v1/central-bank/ustreasury/latest" \
  -H "Authorization: Bearer art_live_your_key"

On paid plans, the table in force on any past date — an invoice date, an entry date, a reporting date:

curl "https://allratestoday.com/api/v1/central-bank/hmrc/2026-03-20?source=GBP&target=USD" \
  -H "Authorization: Bearer art_live_your_key"

What people build with this

• Customs and import duty — value goods at the HMRC rate for the month of the declaration, automatically, instead of pasting a number from a PDF each month.
• VAT invoicing — issue and reconcile foreign-currency invoices at the rate HMRC requires for the period they fall in.
• US federal reporting — convert foreign-currency balances at the Treasury reporting rates that agencies, contractors, and grant recipients must all use.
• Audit and restatement — pull the exact table that was in force on a historical date, amendments applied, so a restated figure matches the original filing.
• ERP and accounting integrations — schedule one fetch per period and cache it; a monthly or quarterly rate cannot change under you mid-period.

Plans

• Every plan, including free: the latest table in force, and single pairs from it.
• Paid plans: historical dates, time series, and the publication calendar.

Because a rate is fixed for a month or a quarter, one cached fetch per period covers every document in it — a small quota goes a very long way here. See plans.

Frequently asked questions

What is a tax authority exchange rate API?

An API that returns the exchange rates national tax and customs authorities publish for compliance — such as HMRC’s monthly customs and VAT rates or the US Treasury’s quarterly reporting rates — rather than live market rates. These are the rates declarations, invoices, and official reports are required to use.

Which tax authorities are covered?

2: HM Revenue & Customs, U.S. Department of the Treasury. More are being added, and the same API also covers 35 central banks.

How is this different from the central bank rates API?

It is the same API and the same endpoints — only the publisher differs. Central banks publish a rate every business day as part of monetary policy; tax authorities fix one rate per month or quarter specifically so that filings within that period agree. Which one you need depends on the rule you are complying with.

Which rate should I use for a customs declaration?

The tax authority’s rate for the period the declaration falls in — not the market rate on the day. Requesting any date inside a period returns that period’s official rate, so a lookup by invoice or entry date resolves to the rate you are required to apply.

How are mid-quarter amendments handled?

The US Treasury amends individual currencies mid-quarter, so its newest publication date is often a handful of rows rather than a table. The API returns the table in force — the quarter’s rates with any later amendments applied — and every row carries the date it was actually published.

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