How is an ABN check digit calculated?

Subtract 1 from the first digit, multiply each of the 11 digits by the weights 10,1,3,5,7,9,11,13,15,17,19, sum the products, and the ABN is valid if that sum is divisible by 89.

How many digits is an ABN?

An ABN is exactly 11 digits. The tool ignores spaces, so you can paste it grouped as 2-3-3-3 (e.g. 51 824 753 556) and it still validates correctly.

Does a valid checksum mean the business is registered?

No. A valid checksum only confirms the number is internally consistent. To confirm a business is currently registered and active, use the official ABN Lookup service. This tool performs no lookup.

What is the difference between an ABN and an ACN?

An ACN is a 9-digit company identifier issued by ASIC. An ABN is 11 digits and is issued to all business types. For companies, the last 9 digits of the ABN are usually the ACN.

Why does my valid-looking ABN fail?

A failed check almost always means a digit was mistyped or transposed. Re-check all 11 digits against the source document; a single wrong digit breaks the mod-89 test.

Australia ABN Validator

An Australian Business Number (ABN) is the unique 11-digit identifier the Australian Business Register (ABR) issues to every business in Australia. Before trusting an ABN on an invoice or supplier record, you can confirm it is well-formed using the official checksum. This validator runs that exact check entirely in your browser, which is useful for procurement, accounts-payable and developers building invoicing software.

How it works

The ABR defines a weighted modulus-89 check 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).

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 becomes 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 remainder 0, so the checksum is valid.

Notes

A valid checksum confirms the number is well-formed, not that the business is registered or active. Use the official ABN Lookup service to confirm registration status. Everything here runs locally — your ABN never leaves your device.