ABN Validator
Check whether an Australian Business Number is valid
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What this check actually proves
An ABN carries its own error detection. Subtract one from the leading digit, multiply the eleven digits by the weights 10, 1, 3, 5, 7, 9, 11, 13, 15, 17 and 19, and the total divides evenly by 89. This tool runs that calculation, which is enough to catch the mistakes people actually make: a digit typed twice, a digit dropped, a pair of digits swapped, an ABN pasted with its ACN by accident.
What it cannot tell you is whether the business exists. A checksum is arithmetic, not a registry lookup. Roughly one in 89 random eleven digit numbers passes it by chance, so a valid result means the number is well formed, not that anybody is trading under it, that the business is registered for GST, or that the entity is still active. For that you need the ATO's ABN Lookup service, which is the authoritative source and the only place the entity name, status and GST registration date live.
The ACN hiding inside a company ABN
When the holder is a company, its ABN is its nine digit Australian Company Number with a two digit prefix in front. The prefix is not arbitrary: it is the value that makes the checksum work, which means an ACN determines its company's ABN exactly. Where that relationship holds, this tool shows the ACN alongside the verdict, so you can cross-check a supplier's paperwork without counting digits by hand. Sole traders, partnerships and trusts hold ABNs without an ACN, so the line only appears when the arithmetic supports it.
Checking ABNs in your own code
Validate on entry, store the digits without spaces, and print them grouped 2-3-3-3 the way the ATO does. Strip spaces, dashes and any leading "ABN" before you check, because that is how people paste them. Reject a leading zero: no ABN starts with one. If you need well formed numbers for your test fixtures, the ABN generator produces them by the batch. Nothing you type here is uploaded, logged or sent anywhere; the whole check happens in this page.
Frequently Asked Questions
How is an ABN checksum calculated?
Subtract one from the first of the eleven digits, then multiply the digits by the weights 10, 1, 3, 5, 7, 9, 11, 13, 15, 17 and 19 and add the results. The number is well formed when that total divides evenly by 89. The subtraction at the start is the step most hand-written implementations forget, and it is why an ABN never begins with zero.
Does a valid checksum mean the business is real?
No. The checksum is arithmetic performed on the digits themselves, so about one in 89 random eleven digit numbers passes it by chance. It tells you the number was typed correctly, not that the entity exists, is trading, or is registered for GST. The ATO ABN Lookup service is the authoritative source for those questions and the only place the entity name and status are published.
How do I get the ACN out of an ABN?
For a company, the ABN is the nine digit ACN with a two digit checksum prefix in front, so the ACN is simply the last nine digits. This tool shows it whenever the arithmetic supports that reading. Sole traders, partnerships and trusts have an ABN with no ACN behind it, so the line does not always appear.
Why is my ABN rejected when it looks correct?
Almost always a transposed pair of digits or a digit read wrong off a printed invoice, which is exactly what the checksum is designed to catch. Also check that you have eleven digits rather than the nine of an ACN, and that no character other than digits and spaces has crept in when copying from a PDF.
Is the ABN I type here sent anywhere?
No. The check is a few lines of arithmetic that run in your browser, with no network request as you type and nothing stored between visits. That is true of every tool on this site.