How do I get the ACN from an ABN?

For a company, the 9-digit ACN is simply the last 9 digits of the 11-digit ABN. The ATO prepends a 2-digit prefix to the ACN and adds its own mod-89 check to form the ABN, so dropping those first two digits recovers the ACN.

How is the ACN check digit calculated?

ASIC weights the first 8 digits of the ACN by 8,7,6,5,4,3,2,1, sums the products, takes the remainder modulo 10, and the complement (10 minus that remainder, modulo 10) must equal the 9th digit.

Does every ABN have an ACN?

No. Only ABNs issued to registered companies map to an ACN. Sole traders, partnerships and trusts have an ABN but no ACN, so the derived 9 digits are only meaningful when the ABN belongs to a company.

Does a valid checksum mean the company is registered?

No. A valid checksum only confirms the number is internally consistent. It does not confirm the entity is currently registered or active — use the official ABN Lookup or the ASIC company register for that.

Is my data sent anywhere?

No. Both checksum calculations run entirely in your browser. The numbers you enter never leave your device and no external lookup is performed.

Australia ABN/ACN Cross-Reference Tool

In Australia a company has both an ABN (Australian Business Number, 11 digits) and an ACN (Australian Company Number, 9 digits), and the two are mathematically linked. This cross-reference tool extracts the ACN from an ABN, then validates each number against its own official checksum — useful in procurement, supplier onboarding and invoicing where you need to confirm an ABN and ACN belong together.

How it works

The relationship and both checks are simple to verify:

Derive the ACN. For a company, the ACN is the last 9 digits of the ABN. The ATO forms the ABN by prefixing the ACN with two digits and adding a mod-89 check.
Validate the ABN. Subtract 1 from the first digit, weight all 11 digits by 10,1,3,5,7,9,11,13,15,17,19, sum the products, and the ABN is valid when the sum is divisible by 89.
Validate the ACN. Weight the first 8 digits by 8,7,6,5,4,3,2,1, sum them, and the complement (10 - (sum mod 10)) mod 10 must equal the 9th digit.

Example and notes

For the ABN 51 824 753 556, the last 9 digits give the candidate ACN 824 753 556. The tool runs the ATO mod-89 check on the full ABN and the ASIC mod-10 check on the derived ACN, reporting both results so you can confirm the pair is consistent.

Remember that the derived ACN is only meaningful when the ABN belongs to a registered company — sole traders and trusts have no ACN. And a passing checksum confirms internal consistency only, not that the entity is currently active; the official ABN Lookup and ASIC register remain the source of truth. All processing is local.