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ABN format checker

Type an ABN (or paste a list of them) and this tool runs the same check-digit algorithm the Australian Business Register publishes, showing the weighted sum so you can see exactly why a number passes or fails.

Check an ABN

Spaces and hyphens are ignored. 11 digits required.
Useful for checking a supplier list. Nothing is uploaded.

The algorithm, step by step

This is the exact procedure published by the Australian Business Register:

  1. Take the 11 digits of the ABN as a string of digits.
  2. Subtract 1 from the first (left-most) digit to get a new 11-digit number.
  3. Multiply each digit by its weighting factor — 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 — by position.
  4. Add the eleven products together.
  5. Divide the total by 89 and note the remainder.
  6. If the remainder is zero, the ABN is correctly formed.

The check digits are the first two digits, so changing almost any single digit in an ABN will make the sum fail. That is the point: it catches typos and invented numbers. The calculator above shows the whole weighted table, so a failed check tells you how it failed, not just that it did.

Worked example

For 51 824 753 556: subtract 1 from the 5 → 41 824 753 556; the weighted sum is 534; 534 ÷ 89 = 6 remainder 0, so the ABN is valid. That example is the one on the Format of the ABN page, and it is one of the cases in this site's test suite.

What this check cannot tell you

A valid format is not a valid business. This tool cannot see the Australian Business Register. An ABN that passes here may be cancelled, may belong to a different entity to the one invoicing you, or may not be registered for GST at all.

Before you rely on it — paying a subcontractor, claiming a GST credit, or using it on your own tax invoice — look the ABN up on ABN Lookup, which shows the registered name, ABN status, GST registration and the date it was registered.

For consistency we also suggest checking that the name on the invoice matches the entity name on ABN Lookup. That is the check that catches most invoice fraud, and no arithmetic can do it for you.

Where an ABN is required on an invoice

A tax invoice must show the supplier's ABN to be valid, and a GST credit generally depends on having a valid tax invoice. If you are buying from a supplier who cannot quote a valid ABN, the ATO's tax invoice rules and the no-ABN withholding rules are the pages that matter — this tool is a starting point, not the authority.

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Common questions

What makes an ABN valid?
An ABN is 11 digits: a 9-digit identifier with two leading check digits. The check digits are calculated using a modulus 89 (remainder after dividing by 89) calculation which is published by the Australian Business Register. This tool performs that arithmetic. It confirms the number is correctly formed — it does not confirm the ABN is registered to anyone.
How do I know if an ABN is actually registered?
Use ABN Lookup at abr.business.gov.au. The check-digit test can only tell you a number is the right shape. An ABN that passes can still be cancelled, and a business can display someone else's ABN. ABN Lookup shows the registered entity name, ABN status and GST registration.
Should I check an ABN before paying a subcontractor?
Yes. If you are paying a contractor and they do not quote a valid ABN, you may have to withhold tax from the payment at the top rate under the no-ABN withholding rules. The check here is a first pass; ABN Lookup is the authoritative check, and it also shows whether the entity is registered for GST, which affects the invoice.
Can I claim GST credits on an invoice with an invalid ABN?
No. A tax invoice must show the supplier's ABN to be valid, and you generally need a valid tax invoice to claim a GST credit. If the ABN fails the check, ask the supplier for a corrected invoice before paying.
Why does 51 824 753 556 pass?
It is the worked example published by ABN Lookup. Subtracting 1 from the first digit gives 41 824 753 556; multiplying each digit by the weights 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 and adding gives 534; and 534 ÷ 89 = 6 exactly, with no remainder.