ABN Validator

What is an ABN?

An ABN (Australian Business Number) is an 11-digit number that identifies a business entity to the government and the community. It is issued by the Australian Business Register (ABR) and used on invoices, tax documents, and business correspondence.

Every business operating in Australia should have an ABN. Sole traders, companies, partnerships, trusts, and non-profit organisations can all have one. The application is free and takes about 15 minutes at abr.business.gov.au.

Your ABN must appear on every invoice you issue. Without it, the payer is required by law to withhold 47% of the payment under the no-ABN withholding rules.

How ABN validation works

The ATO uses a weighted checksum algorithm to validate ABNs. Here is how it works:

1. Subtract 1 from the first digit
2. Multiply each resulting digit by its weighting factor (10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19)
3. Sum all the products
4. Divide the sum by 89 — the remainder must be zero for the ABN to be valid

This tool uses the same algorithm. A valid checksum means the number structure is correct, but does not confirm that the ABN is currently registered or active. To check registration status, visit the Australian Business Register.