Free calculator
ABN and ACN checker
Type an ABN or ACN to check its format and check digits, right here in your browser. To see who it belongs to and whether it’s active, use the government’s ABN Lookup.
11 digits; spaces are fine
Check
Valid ABN
- As printed on an invoice
- ABN 51 824 753 556
- Check digits
- Match
The check digits only show the number is well formed. Use ABN Lookup to see whether it’s registered and active.
Make an invoice with your ABNHow an ABN is checked
An ABN has 11 digits, printed in groups as 51 824 753 556. The first two are check digits: subtract 1 from the first digit, multiply each digit by 10, 1, 3, 5, 7, 9, 11, 13, 15, 17 and 19, and add them up. A valid ABN’s total divides exactly by 89.
An ACN has 9 digits, like 004 085 616. The last digit is a check digit worked out from the first eight.
A number that passes is well formed, not necessarily registered or active — a typo can only be caught when the digits don’t add up.
ABN and ACN on invoices
- Every tax invoice must show your ABN.
- Even without GST, quote your ABN: otherwise your client must withhold 47% of the payment.
- A company shows its name and ACN — or its ABN, when the ABN’s last nine digits are the ACN.
Last updated 2026-10-01. This calculator is a guide, not tax advice.
Frequently asked questions
Does this tell me who owns an ABN?
No. It only checks the number’s format and check digits, offline. ABN Lookup (abr.business.gov.au) shows the business name and whether it’s registered for GST.
Why do some ABNs in examples fail?
Made-up numbers such as 36 123 456 789 don’t pass the check digits. Real ABNs always do.
Is my ACN inside my ABN?
For a company, usually yes: the ABN is two check digits followed by the 9-digit ACN.