International · Validation
Australian Business Number validator.
Check an ABN against the real modulus-89 check-digit algorithm the Australian Business Register publishes — the same math the ABR's own Help page walks through, step by step.
How it works: subtract 1 from the first digit, multiply all 11 digits by fixed weights (10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19), sum the products, and divide by 89 — a remainder of zero means the ABN is shape-valid.
Inputs
Result
Enter an ABN and press Check.
How the ABN check digit works
Every Australian business entity gets an 11-digit Australian Business Number (ABN), issued by the Australian Business Register (ABR) on behalf of the ATO. Unlike Canada's CRA Business Number, the ABN carries a real, published check-digit algorithm — a modulus-89 checksum — so a typo can be caught mathematically, not just by shape.
| Step | What happens |
|---|---|
| 1 | Subtract 1 from the first (left-most) digit |
| 2 | Multiply each of the 11 digits by its position weight: 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 |
| 3 | Sum the 11 products |
| 4 | Divide the sum by 89 and take the remainder |
| 5 | A remainder of zero means the ABN is valid |
The ABR's own Help page walks through a worked example, 51 824 753 556: the first digit becomes 5−1=4, the weighted products sum to 534, and 534÷89=6 exactly — a zero remainder, so the ABR states it as valid. This tool reproduces that exact arithmetic.
Format check, not an ABR Lookup
A remainder of zero confirms the 11 digits are internally consistent with the ABR's checksum — it does not confirm the number was ever actually issued to a real entity, or that the entity is still active. Only the ABR's own ABN Lookup service can confirm that.
Checksum only — not an ABR lookup
This tool checks the modulus-89 checksum entirely in your browser. It does not query the ABR or ATO, and a checksum-valid result is not proof the number was ever issued. Nothing you type is transmitted, stored, or logged.