# Format of the ABN

The Australian Business Number (ABN) is a unique 11 digit identifier issued to all entities registered in the Australian Business Register (ABR).

The 11 digit ABN is structured as a 9 digit identifier with two leading check digits. The leading check digits are derived using a modulus 89 (remainder after dividing by 89) calculation.

To verify an ABN:

- Subtract 1 from the first (left-most) digit of the ABN to give a new 11 digit number
- Multiply each of the digits in this new number by a "weighting factor" based on its position as shown in the table below
- Sum the resulting 11 products
- Divide the sum total by 89, noting the remainder
- If the remainder is zero the number is a valid ABN

For example, to check if **51 824 753 556** is a valid ABN:

- Subtract 1 from the first (left-most) digit (5) to give
**41 824 753 556** - Multiply each of the digits in
**41 824 753 556**by the "weighting factor" based on its position as shown in the table below - Sum (Digit * weight) to give a total of 534
- Divide 534 by 89 giving 6 with zero remainder.
- As the remainder is zero,
**51 824 753 556**is a valid ABN.

Digit | Postion | Weighting | Digit * weight |
---|---|---|---|

4 | 1 | 10 | 40 |

1 | 2 | 1 | 1 |

8 | 3 | 3 | 24 |

2 | 4 | 5 | 10 |

4 | 5 | 7 | 28 |

7 | 6 | 9 | 63 |

5 | 7 | 11 | 55 |

3 | 8 | 13 | 39 |

5 | 9 | 15 | 75 |

5 | 10 | 17 | 85 |

6 | 11 | 19 | 114 |

Total: |
534 |

Try your own ABN validation: