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Calculating ABN from ACN number

If you have an ACN (Australian Company Number) you may be waiting for an ABN (Australian Business Number).
Sometimes you may be pressed by an entity to provide the ABN for the entity.


Short answer is: search the ABR or apply for your ABN (or chase up your accountant who is doing so for you).


Over 10 years ago – I had a software designer calculate what the ABN would be from an ACN number.

Cleverly, there is a particular formula that only one number out of the 99 digits will work with the ACN number.

The ACN number itself (and TFNs as well) are calculated cleverly to avoid fraud as they are not sequential, they have a log so that it is difficult to predict which the next number will be.


I still have the ACN to ABN spreadsheet – and very occasionally I provide the ABN number to clients for specific circumstances.

I toyed with the idea of putting it on my website either for a fee or as a download – but then I decided that it would more likely to be used for evil than good so I’ve not done so.


However, nowdays there are many entities that are able to search the ABR (Australian Business Register) – with the ABN that you enter into a form.
This means that simply having the number is no longer good enough – you need to be registered.


If you have your ABN but not your ACN – the ACN is the 9 digits after the initial 2 digits

If you have your ACN but not your ABN – you can search the Australian Business Register at


When you are in CHROME and you type the address (I just type ABR and it completes the rest for me) – before I presss ENTER I press TAB – and then it comes up with a new menu saying “SEARCH AUSTRALIAN BUSINESS REGISTER” – and then I type the business name that I am looking for the ABN for.


(or less often the ABN of an entity that I suspect is belonging to a different entity – for example when a business puts an ABN with a trustee company – the ABN will usually show that of the Trust).

About Scott Kay

Integrity Plus Accounting

Scott Kay is currently a husband and father of one young daughter. He founded Integrity Plus Accounting over 4 years ago and has had his own clients for over 10 years. He has worked for 3 other accounting businesses and loves to help clients grow – in their business, their wealth, their mindset and their lives.