How the ABN check digit works
ABNs include a built-in check, so typos can be caught without looking anything up. The formula published by the ABR is:
- Subtract 1 from the first (left) digit.
- Multiply each of the 11 digits by its weight: 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19.
- Add the results.
- Divide the total by 89. If there is no remainder, the ABN is valid.
Example: 51 824 753 556 → (5−1)×10 + 1×1 + 8×3 + 2×5 + 4×7 + 7×9 + 5×11 + 3×13 + 5×15 + 5×17 + 6×19 = 534 = 89 × 6, so it is valid.
A valid check digit only means the number is well-formed — not that it is issued or active. Use the ABN checker and then look the ABN up.