How to use the abn and acn validator
- Paste the numbers, one per line. Spaces between groups of digits are fine.
- An 11 digit line is read as an ABN, a 9 digit line as an ACN and an 8 digit line as an ACN without its last digit.
- Read whether each check works, the weighted sum and the detail column.
- Copy the table or save the CSV.
Formula
ABN: subtract 1 from the first digit, then sum = 10 d1 + 1 d2 + 3 d3 + 5 d4 + 7 d5 + 9 d6 + 11 d7 + 13 d8 + 15 d9 + 17 d10 + 19 d11
ABN is valid when sum mod 89 = 0
ACN: sum = 8 d1 + 7 d2 + 6 d3 + 5 d4 + 4 d5 + 3 d6 + 2 d7 + 1 d8, check digit = (10 - sum mod 10) mod 10
The ABN rule is a weighted sum modulo 89. The ACN rule uses weights from 8 down to 1 on the first eight digits and a final digit that rounds the sum up to a multiple of 10.
Checking 51 824 753 556
Subtract 1 from the first digit to get 4, 1, 8, 2, 4, 7, 5, 3, 5, 5, 6. Multiplying by 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 gives 40, 1, 24, 10, 28, 63, 55, 39, 75, 85 and 114. The total is 534, which is 6 x 89, so the ABN passes.
For the ACN 004 085 616 the first eight digits 0, 0, 4, 0, 8, 5, 6, 1 times 8 down to 1 give 0, 0, 24, 0, 32, 15, 12, 1, a total of 84. The check digit is (10 - 4) mod 10 = 6, which matches the ninth digit.
What a passing number means
The check digits catch most typing mistakes, such as one wrong digit or two swapped digits. A number that passes has valid arithmetic and nothing more.
It does not show that a business is registered or active. Use the public ABN Lookup site to confirm a registration.
Why the ABN subtracts one from the first digit
The published rule subtracts 1 from the first digit before the weights are applied, and the weighted sum is then tested against 89.
The page applies that step to whatever first digit you enter, so a number that starts with 0 is worked out too.
Assumptions and limits
- Only the check digit arithmetic is tested. Registration status and names are not looked up.
- An 11 digit number is treated as an ABN and a 9 digit number as an ACN.
- Spaces and hyphens are ignored. Other characters are rejected.
- Nothing you type is stored or sent anywhere.
- Up to 200 lines are read at once.
Frequently asked questions
What is the difference between an ABN and an ACN?
An ABN is an 11 digit number used for tax and business dealings. An ACN is a 9 digit number given to a company when it is registered. A company usually has both, and its ABN is built from its ACN.
Why does a number I know is real fail?
Most often a digit was mistyped or two were swapped. Check the number against the registration document. If it still fails, make sure you did not drop a leading zero.
Can I calculate a missing ACN digit?
Yes. Enter the first eight digits on a line and the page adds the ninth digit that makes the check work.
Is this the same as looking the number up?
No. Looking a number up confirms who holds it. This page only tests whether the digits follow the arithmetic rule, entirely in your browser.