Business Australia
ABN Validator (and ACN Check)
Every Australian Business Number carries a built-in check: weight the digits, add them up, and the total must divide by 89. This validator runs that test, and the 9-digit ACN test too, shows the arithmetic, and keeps what you type in your browser.
Free, no signup Runs in your browser
11 digits for an ABN or 9 for an ACN. Spaces are fine. Checked in your browser — nothing is sent anywhere.
Result
—Type
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Formatted
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Weighted total
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Check
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The working, digit by digit
How do I check if an ABN is valid?
An ABN is valid when you subtract 1 from its first digit, multiply the eleven digits by 10, 1, 3, 5, 7, 9, 11, 13, 15, 17 and 19, add the results, and the total divides exactly by 89. That proves the number is well formed; it does not prove the business exists. ABN Lookup confirms registration.
What passing does and does not show
The check digits are a defence against typing mistakes, and they do that job well: a transposed pair or a single wrong digit almost never produces another number that also passes. They say nothing about whether the number was ever issued, whether the business is still trading, or whether the ABN is active or cancelled.
To confirm any of those, the only authoritative place is ABN Lookup, run by the Australian Business Register. Use this page to catch the typo before you search, or to sanity-check a field in a form or spreadsheet.
ABN or ACN
An ABN has 11 digits and identifies a business to the tax system. An ACN has 9 digits and identifies a company to ASIC. A company usually has both, and its ABN is often its ACN with two check digits placed in front, but the two rules are different, which is why this tool picks the right one from the number of digits you enter.
If you are putting an ABN on an invoice, the tax invoice generator on this site checks the number with the same rule as you type.
How this is calculated
The Australian Business Register publishes the ABN rule; ASIC publishes the ACN rule. Both are short.
ABN: subtract 1 from the first digitmultiply the eleven digits by the weights 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19 and add themthe ABN is well formed when that total divides exactly by 89ACN: multiply the first eight digits by 8, 7, 6, 5, 4, 3, 2, 1 and add themthe ninth digit must equal 10 minus the last digit of that total (counting 10 as 0)
Subtracting 1 from the first digit is what makes the ABN rule work: it guarantees that a number with a leading zero cannot be valid, and it spreads the valid numbers through the range so a single typing error is very unlikely to land on another valid one. Dividing by 89 is the check; the reason 89 was picked is not published, but it is prime, which is the usual property wanted.
A passing result is weaker evidence than it looks. About one eleven-digit number in 89 passes by chance, so well-formed does not mean issued, and a cancelled ABN passes just as a live one does.
A worked example: the ATO's own sample ABN
51 824 753 556 is the ABN the Australian Business Register uses to illustrate the rule. Its digits are 5 1 8 2 4 7 5 3 5 5 6.
- Subtract 1 from the first digit5 becomes 4, giving 4 1 8 2 4 7 5 3 5 5 6
- Multiply by 10 1 3 5 7 9 11 13 15 17 19 and add534
- 534 ÷ 896 exactly, remainder 0
- Change the last digit to 7 (51 824 753 557)total 553, remainder 19
51 824 753 556 passes; changing a single digit breaks it. A 9-digit ACN follows its own rule: 004 085 616 totals 84, so its check digit is 6, which it has.
The same arithmetic runs in the tool above, which also shows the product of every digit so you can find where a typo sits.
Privacy
The number you type is processed in your browser and goes nowhere. There is no lookup, no account and no log. ABNs are public records, but you should not have to tell a third party whose ABN you are checking.
Common reasons a real ABN fails
The usual cause is a mistake copying it: a digit dropped, two digits swapped, a letter O typed for a zero, or a trailing digit lost when a spreadsheet converted the cell to a number. If a number you were given fails, check it against the source document before assuming it is false.
What this calculator does not do
- Shows a number is well formed, not that it is registered or active
- Does not look numbers up — ABN Lookup is the authority
- Does not validate a TFN; that is deliberately not built because TFNs are private
- Checks the ACN digit only, not whether the company exists
Common questions
How can I check if an ABN is valid?
Subtract 1 from the first digit, weight the eleven digits 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, add them, and the total must divide by 89. This page does it for you and shows the steps.
Does a valid result mean the business exists?
No. It shows the number is well formed. Whether it is registered, active or cancelled is only shown on ABN Lookup.
How many digits does an ABN have?
Eleven. An ACN, the number ASIC gives a company, has nine.
Why does a genuine ABN fail the check?
Almost always a typing or copying error — a swapped or missing digit, or a leading zero lost in a spreadsheet. Re-enter it from the source document.
Is the number I type sent anywhere?
No. The check runs in your browser and nothing is uploaded, stored or logged.
Where these numbers come from
- Australian Business Register — ABN format The ABN check-digit rule and the ATO's own example number
- ASIC — Australian company number digit check How the ACN's ninth digit is derived
- ABN Lookup Where to confirm that a number is registered and active
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
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