# Format of the ABNVersion: 9.8.18

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:

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

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

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