Australia ABN validator
How the ABN format works
- Format
- 11 digits
- Example
- 51824753556
Things to watch for
- 11 digits with a weighted mod-89 check
- Checksum only — use ABN Lookup to confirm registration and GST status
- Do not confuse with the 9-digit ACN
^[0-9]{11}$An Australian Business Number (ABN) is the 11-digit identifier the Australian Business Register issues to every business, sole trader, partnership, and company operating in Australia, used on invoices, tax documents, and business registrations — for example 51 824 753 556. If you’re entering a supplier’s ABN into accounting software, checking an invoice before paying it, or validating a signup form, this tool catches a mistyped digit before it goes any further.
The 11 digits carry a built-in check: a weighted mod-89 calculation across all 11 positions, with the first digit adjusted before weighting. That’s what lets a validator catch a single mistyped or transposed digit without contacting the Australian Business Register at all — the same underlying idea as a credit card checksum catching a typo before a payment is even submitted.
How this validator works
This tool checks that the input is 11 digits and runs the real weighted mod-89 formula the Australian Business Register uses to generate valid ABNs, entirely in your browser. Nothing is sent to a server, and there’s no signup.
What a pass doesn’t tell you
A pass confirms the number is mathematically well-formed — not that it’s currently registered, GST-active, or attached to a trading business. ABNs get cancelled when a business closes and aren’t reissued to someone else, so a checksum pass on an old invoice doesn’t guarantee the entity is still operating. For current registration and GST status, the Australian Business Register’s ABN Lookup service is the authoritative source. Don’t confuse an ABN with the 9-digit ACN a company is assigned at incorporation — the two overlap in most company ABNs but aren’t interchangeable.
Scope: this page and tool cover ABN format and checksum validation only — not registration status, GST status, or ACN lookup.
Content last reviewed 2026-07-07.
ABN FAQ
What's the difference between an ABN and an ACN?
An ACN (Australian Company Number) is the 9-digit identifier ASIC assigns when a company is incorporated. An ABN (Australian Business Number) is the broader 11-digit identifier — for a company it's usually the ACN with 2 extra digits prefixed, but sole traders, partnerships, and trusts can hold an ABN without ever having an ACN.
Does a valid ABN checksum mean the business is registered for GST?
No. Passing the checksum only confirms the 11 digits are mathematically well-formed. GST registration is a separate, changeable status, so confirming whether a business is currently GST-registered requires looking the ABN up directly rather than relying on the checksum alone.
Where can I check if an ABN is currently active?
The Australian Business Register's ABN Lookup service shows an ABN's registered entity name, GST status, and whether the registration is still active — information a checksum alone can't provide.