VAT Number Formats & Checksums: All 33 Countries

Every VAT or GST number a business hands you is a string that either matches a country's format or it does not. That format check, a regular expression plus sometimes a checksum digit, is the first and cheapest thing any validator can do: no network call, no upstream registry, just a pattern match against the digits the customer typed. It is also the most misunderstood part of VAT validation, because passing the regex is not the same thing as being a real, active registration.

This is the developer reference for that distinction across all 33 country prefixes Avatcado supports: the 27 EU member states, Northern Ireland (XI), the United Kingdom (GB) post-Brexit, Switzerland (CH) and Liechtenstein (LI), Norway (NO), and Australia (AU). Every pattern, local name, and checksum below is pulled directly from the same VAT_FORMAT_RULES table the Avatcado API itself validates against, so this table cannot silently drift out of sync with what the API actually accepts.

VAT number format reference: all 33 countries

"Checksum" below means the country embeds a check digit or check letter in the number itself, one that can be verified by arithmetic alone. Three formats in this table, Romania, Slovakia, and Sweden, have no checksum: any string of the right length and character set passes the format check, so a registry lookup is the only way to tell a real number from a made-up one.

Every row links to that country's own page, which adds the check-digit description in full, a real registered example number, and a free live checker against the actual registry.

How the checksums work: three worked examples

Most checksums in the table above follow the same shape: multiply each digit by a fixed weight, sum the results, and take the sum modulo some base. The base and weights differ per country, but three of Avatcado's own checksum functions (all MOD 11 or MOD 89 style, and all documented above) are simple enough to walk through by hand.

Switzerland (CH): weighted MOD 11

A Swiss UID like CHE-105.836.495 validates as CH105836495. Drop the CH prefix and MWST suffix to get nine digits: 105836495. Multiply the first eight, 1, 0, 5, 8, 3, 6, 4, 9, by the weights 5, 4, 3, 2, 7, 6, 5, 4: that gives 5 + 0 + 15 + 16 + 21 + 36 + 20 + 36 = 149. Take 149 mod 11 = 6, then 11 - 6 = 5. That is the ninth digit of the real number, so it checks out.

Norway (NO): weighted MOD 11

A Norwegian organization number, NO982463718MVA, strips down to 982463718. The first eight digits, 9, 8, 2, 4, 6, 3, 7, 1, are weighted 3, 2, 7, 6, 5, 4, 3, 2: 27 + 16 + 14 + 24 + 30 + 12 + 21 + 2 = 146. 146 mod 11 = 3, and 11 - 3 = 8, matching the real ninth digit. Same mechanism as Switzerland, different weights.

Australia (AU): weighted MOD 89

The Australian ABN checksum works the same way with a different base and an extra first step. For AU51824753556, subtract 1 from the first digit only (5 becomes 4), then weight all eleven digits 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19: 40 + 1 + 24 + 10 + 28 + 63 + 55 + 39 + 75 + 85 + 114 = 534. A valid ABN divides evenly by 89, and 534 / 89 = 6 exactly.

A remainder-based check like these three catches typos and transposed digits before they ever reach a network call, but it cannot tell you whether a number was ever issued. A string that happens to satisfy the arithmetic is exactly as "checksum-valid" as one that a real tax authority assigned to a real business.

Edge cases the regex alone cannot capture

Format documentation in this reference comes with notes for exactly this reason: some countries have quirks that a plain pattern match will not surface on its own.

Northern Ireland's XI prefix

Same digits as the trader's GB VAT number with XI substituted for GB; it is an add-on to the UK VAT registration, not a separate one XI numbers validate in VIES; GB numbers were removed from VIES on 1 January 2021 A validator that only recognizes GB and the 27 EU prefixes will reject every Northern Ireland goods registration; XI needs to be its own supported prefix, not folded into GB.

Greece: EL, not GR

The prefix is EL (from Ellada/Hellas), not the ISO code GR; GR-prefixed numbers fail VIES

The United Kingdom, post-Brexit

GB numbers are not in VIES since 1 January 2021; validate via HMRC's checker or the Check a UK VAT Number API

See the HMRC VAT Check API guide for what replaced VIES coverage for GB numbers.

Format-valid is not the same as registered

Nothing on this page, or in the VAT_FORMAT_RULES table it is generated from, can tell you whether a number was ever issued to a real business, or whether that business is still active. Format checking rules out obvious typos and malformed input before you spend a network round trip on it. Confirming the number is real is a separate step, one that has to go to the actual registry: VIES for the EU and Northern Ireland, HMRC for the UK, the BFS UID Register for Switzerland and Liechtenstein, the Bronnoysund Register for Norway, and the ABR for Australia. Each of those is a different service with its own uptime, rate limits, and response shape, which is the whole reason to put one API in front of all of them rather than integrating each registry separately.

Avatcado's free VAT checker runs both steps for any of the 33 formats above: format validation first, then a live registry lookup, with no account required. For the same logic wired into your own backend, the how to validate EU VAT numbers programmatically guide covers the REST API end to end, and validating VAT numbers in TypeScript covers the same flow with the typed SDK.

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