An Australian Business Number (ABN) is the unique 11-digit identifier the Australian Business Register issues to every business operating in Australia. Before you trust an ABN on an invoice, quote or supplier record, you can confirm it is internally well-formed using the Australian Taxation Office (ATO) checksum. This free validator runs that exact check in your browser — useful for accountants, developers building invoicing software, and anyone verifying a supplier.
How it works
The ATO defines a weighted modulus-89 check digit algorithm:
- Strip spaces so you have 11 digits.
- Subtract 1 from the first (leftmost) digit.
- Multiply each of the 11 digits by its positional weight: 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19.
- Add the 11 products together.
- The ABN is valid if that sum is exactly divisible by 89 (remainder 0).
The tool shows you the weighted sum and the sum mod 89 remainder so you can see precisely why a number passed or failed.
Example
Validate 51 824 753 556 → digits 5 1 8 2 4 7 5 3 5 5 6.
Subtract 1 from the first digit (5 → 4), then apply the weights:
(4×10) + (1×1) + (8×3) + (2×5) + (4×7) + (7×9) + (5×11) + (3×13) + (5×15) + (5×17) + (6×19) = 534.
534 ÷ 89 = 6 with remainder 0, so the checksum is valid.
| Field | Value |
|---|---|
| Formatted | 51 824 753 556 |
| Weighted sum | 534 |
| Sum mod 89 | 0 (must be 0) |
A valid checksum confirms the number is well-formed, not that the business is registered. To confirm a business is active, use the official ABN Lookup service. Everything here runs locally — your ABN never leaves your device.