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ABN

Tax

Australian Business Number

An 11-digit identifier issued by the Australian Business Register to businesses and organisations, used for GST registration, invoicing, and dealings with government and other businesses.

Definition

An Australian Business Number (ABN) is an 11-digit identifier issued by the Australian Business Register (ABR) to businesses, sole traders, and other organisations operating in Australia. It's required for GST registration, issuing valid tax invoices, and most formal dealings with government agencies and other businesses.

Like the TFN, an ABN includes a built-in checksum that catches many transcription errors before a number is used in an invoice, accounting system, or business registration. The Australia ABN Validator applies this checksum directly.

Formula

The ABN checksum uses a weighted modulo-89 calculation:

  1. Subtract 1 from the first (leftmost) digit.
  2. Multiply each of the 11 digits by its corresponding weight: 10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19.
  3. Sum all eleven weighted products.
  4. The ABN is valid if this sum is evenly divisible by 89.

Worked Example

For the ABN 51 824 753 556 (51824753556):

After subtracting 1 from the first digit: 41824753556

Weighted sum of all 11 digits = 534

534 ÷ 89 = 6 exactly, so the checksum passes, confirming the ABN is correctly formed.

Key Things to Know

  • 11 digits, modulo-89 checksum: distinct from the TFN's 9-digit, modulo-11 algorithm.
  • Required on most business invoices: businesses registered for GST must display a valid ABN on tax invoices.
  • Not the same as an ACN: an ABN identifies a business broadly; an ACN (issued by ASIC) specifically identifies a registered company.
  • A valid checksum isn't proof of active registration: use the official ABN Lookup tool to confirm a business's actual registration status.

Related Terms

TFNAustralia Tax File NumberGSTGoods and Services Tax

Frequently Asked Questions

An ABN is used for GST registration, issuing valid tax invoices, and dealing with government agencies and other businesses. Most Australian business invoices are required to display a valid ABN.
First, subtract 1 from the leftmost digit. Then multiply all 11 digits by their corresponding weights (10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19), sum the products, and the ABN is valid if that sum is exactly divisible by 89.
No — a passing checksum only confirms the number is structurally well-formed, not that it's currently registered or active. Confirming an ABN's actual registration status requires searching the official ABN Lookup tool on the Australian Business Register website.
An ACN (Australian Company Number) is a separate 9-digit identifier issued specifically to companies by ASIC, while an ABN is an 11-digit number issued more broadly to businesses, sole traders, and other entities by the Australian Business Register. A registered company's ABN often embeds its ACN within the number.
ABNs are typically printed on invoices, registration documents, and official business correspondence. You can also search for a specific business's ABN using the free ABN Lookup tool on the Australian Business Register's website.