taxInvoice.appABN validator
Make the invoice
Free PDF, checked for the required fields.
Worked example: 51 824 753 556
Digits after subtracting 1 from the first: 4 1 8 2 4 7 5 3 5 5 6. Weighted sum: 40 + 1 + 24 + 10 + 28 + 63 + 55 + 39 + 75 + 85 + 114 = 534. 534 / 89 = 6 exactly, so the ABN is valid.
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Frequently asked questions
How is an ABN validated?
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, add the results, and check the sum divides exactly by 89.
Does a valid checksum mean the business is registered?
No. It only proves the number is well-formed. Use the official ABN Lookup to confirm the ABN is active and who it belongs to.
What happens if my invoice has no ABN?
Your customer may have to withhold 47% of the payment and send it to the ATO, so always include a valid ABN.