International · Conversion
ACN to ABN converter.
Many Australian companies have an ABN made of two check digits followed by their ACN. Enter 9 digits to see the ABN that would end in them, or 11 digits to split an ABN back into its last nine.
How it works: the ABN's two leading digits are check digits from a modulus 89 sum (the first digit is reduced by 1, then each digit is weighted 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19). Going from nine digits to an ABN means finding the two-digit prefix that makes that sum divisible by 89.
Inputs
Result
Enter a number and press Check.
How an ACN relates to an ABN
An ABN is eleven digits: two leading check digits and a nine digit identifier. ASIC says most companies have an ABN as well as an ACN, and that a company's ABN is often the same as the ACN with two extra numbers at the beginning. Section 1344 of the Corporations Act 2001 lets a body use its ABN where an ACN, ARBN or ARSN would be required only if the last nine digits of the ABN are the same, and in the same order, as that number.
| Step | What happens |
|---|---|
| 1 | Pick a two-digit prefix from 10 to 99 and put it in front: for 53 004 085 616 the prefix is 53 |
| 2 | Subtract 1 from the first digit: 5 becomes 4, giving 43 004 085 616 |
| 3 | Multiply the eleven digits by 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 and add them: the total is 445 |
| 4 | 445 ÷ 89 = 5 with remainder 0, so 53 004 085 616 is a valid ABN |
When two ABNs fit
The two leading digits can only take values that make the weighted sum divisible by 89. When the nine digits leave a remainder of exactly 0, both 10 and 99 in front work arithmetically, and the tool shows both. For every other nine-digit number there is one answer.
A calculation, not a record of what was issued
The tool finds an ABN that is arithmetically valid for those nine digits. It cannot tell whether the Australian Business Register actually issued that ABN to the body that holds the ACN: ASIC says ABNs also go to sole traders and partnerships, which have no ACN to embed, and says only that a company's ABN is often, not always, its ACN with two digits in front. Confirm the real ABN on ABN Lookup.
Sources
- Australian Business Register: Format of the ABN (modulus 89, weights)
- ASIC: Australian Company Number digit check (archived 2024)
- ASIC: Australian Company Number (ABN often the ACN with two extra digits)
- Corporations Act 2001 s 1344: Use of ABN
Retrieved October 2026. Agencies can change these rules; check the issuing authority if a result matters.
FAQ
How do I convert an ACN to an ABN?
Put two check digits in front of the nine-digit ACN so the ABN passes the modulus 89 test. This tool finds them for you. Because a company’s ABN is only often, not always, built that way, confirm the real ABN on ABN Lookup.
How do I find the ACN from an ABN?
For a company, the ACN is usually the last nine digits of the ABN. This tool splits them off and checks whether they pass ASIC’s ACN check digit. ABNs also go to sole traders and partnerships, which have no ACN inside them.
Can I use my ABN instead of my ACN on documents?
Section 1344 of the Corporations Act 2001 allows it where the last nine digits of the ABN are the same, and in the same order, as the ACN, ARBN or ARSN.
Why do some nine-digit numbers give two ABNs?
When the nine digits leave a remainder of 0 in the modulus 89 sum, both 10 and 99 as the leading digits are arithmetically valid. Only the register shows which one, if either, exists.
Is the number I type sent anywhere?
No. The conversion runs in your browser and nothing you type is transmitted, stored or logged.
A calculation, not a register lookup
This tool applies the published check-digit arithmetic in your browser. It does not query the Australian Business Register or ASIC, and a converted number is not proof that the ABN was issued. To confirm it, look it up on ABN Lookup. Nothing you type is transmitted, stored or logged.