ABN Australian

What is the ABN?

The Australian Business Number (ABN) is an 11-digit identifier issued by the Australian Taxation Office (ATO) through the Australian Business Register (ABR). Every business that deals with the ATO, registers for GST or invoices other businesses needs one.

The ABN is public information: anyone can look up a business name, status and registration details through the free ABN Lookup service. It is usually displayed in the grouped format XX XXX XXX XXX.

How the checksum works

The ABN carries a built-in check that catches most typos. The algorithm is:

• Subtract 1 from the first digit.
• Apply the weights [10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19] across the 11 digits.
• Multiply each digit by its weight and sum the results.
• The ABN is valid only if the total is exactly divisible by 89 (sum mod 89 == 0).

ABN vs ACN vs TFN

Australia uses several identifiers that are easy to confuse:

• ABN — 11 digits, identifies any business entity for tax and GST purposes.
• ACN (Australian Company Number) — 9 digits, identifies registered companies only.
• TFN (Tax File Number) — private personal/entity tax number, never shared publicly.

Common pitfalls

• Confusing the 11-digit ABN with the 9-digit ACN — a company's ABN often contains its ACN as the last 9 digits.
• Forgetting to subtract 1 from the first digit before applying the weights.
• Treating spaces in XX XXX XXX XXX as significant — strip all non-digits first.
• Assuming a valid checksum means the ABN is active — only ABN Lookup confirms current status.

FAQ

Is the ABN confidential? No. The ABN and the entity it belongs to are public and searchable through ABN Lookup.

Does a valid checksum guarantee the business exists? No. The checksum only proves the number is well-formed; you still need ABN Lookup to confirm the business is registered and active.

Why divide by 89? 89 is the prime modulus chosen for the ABN scheme so that single-digit errors and most transpositions change the remainder and are detected.