How to verify an Australian Business Number (ABN)

The Australian Business Register says an ABN is a unique 11-digit identifier issued to entities registered in the ABR. It is structured as a 9-digit identifier with two leading check digits, derived using a modulus 89 calculation.

Look it up on ABN Lookup

The official search is ABN Lookup. Enter the ABN, or search by name, to see the registered entity name and the ABN status (for example, whether it is active or cancelled). This is the step that tells you whether a number that looks valid is actually still registered to the business in front of you.

When you check a subcontractor, compare the name and ABN on the invoice, contract or insurance certificate with the name and status on ABN Lookup. A cancelled ABN, or an ABN that belongs to a different entity, is a reason to stop and ask for the correct details before work starts.

The official checksum

ABN Lookup publishes this method to verify the format of an ABN:

1. Subtract 1 from the first (left-most) digit of the 11-digit ABN, to give a new 11-digit number.
2. Multiply each digit in this new number by a weighting factor for its position. The weights, from first digit to last, are 10, 1, 3, 5, 7, 9, 11, 13, 15, 17 and 19.
3. Add those 11 products together.
4. Divide the total by 89 and note the remainder.
5. If the remainder is zero, the number is a valid ABN format.

ABN Lookup’s own example is 51 824 753 556. After subtracting 1 from the first digit, the weighted total is 534, and 534 ÷ 89 has a remainder of zero, so the number is a valid ABN.

A checksum only tells you the number is a well-formed ABN. It does not tell you who owns it, or whether it has been cancelled. For that, use ABN Lookup or Datesly’s free ABN checker, which runs the official checksum in your browser and can look the number up on the ABR when that service is connected. The tool does not store the number you enter.