ABN Lookup & Checker

Check any Australian Business Number in a second — the same check-digit rule the ATO uses, free and with no signup.

What is an ABN?

An Australian Business Number is the unique 11-digit identifier every business dealing with the ATO uses — on invoices, when registering for GST, and when dealing with other businesses. It identifies your business publicly; it’s not a secret and it’s not the same as your ACN or TFN.

Why check an ABN before paying an invoice?

If a supplier doesn’t quote a valid ABN on an invoice, you may be required to withhold 47% of the payment (“no-ABN withholding”). A quick check also catches typos and confirms the business actually exists and whether it’s registered for GST — which decides whether it can charge you GST at all.

How ABN validation works

ABNs aren’t random: the digits carry a built-in checksum. Subtract 1 from the first digit, multiply each of the 11 digits by its official weighting (10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19), and add the results — for a genuine ABN the total divides evenly by 89. That’s the exact rule this checker applies, so a “valid” result means the number is structurally real, and the register lookup confirms who it belongs to.

ABN vs ACN — which one goes on an invoice?

An ACN (Australian Company Number, 9 digits) is issued by ASIC to companies only; the ABN (11 digits) is issued by the ATO to any business, including sole traders. Invoices need the ABN — for companies the ABN is usually the ACN with two extra digits in front.