How is an ABN checksum 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 the sum is divisible by 89.

Does a valid checksum mean the business is registered?

No. A valid checksum confirms the ABN is internally consistent, not that the business is currently registered or active. Use the official ABN Lookup to confirm that. This tool performs no lookup.

Is my ABN sent anywhere?

No. The checksum is computed entirely in your browser — privacy-first, the ABN never leaves your device.

Australian ABN Validator

Australian Business Number (ABN) validator

Verify an 11-digit Australian Business Number using the official ATO mod-89 checksum. The algorithm subtracts 1 from the first digit, multiplies each of the 11 digits by the positional weights 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, sums the products, and treats the ABN as valid when that sum is divisible by 89.

This is privacy-first: the calculation runs locally with no ABN Lookup call. A valid checksum means the number is well-formed — to confirm the business is registered and active, use the official ABN Lookup service.