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ABN or ACN on your company stamp: which number goes where
5 September 2026 ยท Estamplab
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Australia gives a company two numbers and asks for both in different places, which is why this is the most common mistake on an Australian company stamp. They are not interchangeable and they do not belong in the same spot.
Open the stamp maker and start from any of these
For whether an Australian company needs a common seal at all, that is answered in stamp maker Australia. This page is only about the numbers.
ABN and ACN: what each one is
The ACN is the Australian Company Number, nine digits, issued by ASIC when the company is registered. It identifies the company as a legal entity. Only companies have one: a sole trader or a partnership does not.
The ABN is the Australian Business Number, eleven digits, issued by the ATO. It identifies the business for tax, and anybody carrying on an enterprise can have one, including sole traders and trusts. Where a company has both, the ABN is normally the ACN with two digits added on the front.
So: ACN answers "which registered company is this", ABN answers "which taxpayer is this".
Which number goes on which stamp
On an invoice stamp: the ABN. Tax invoices are the document that has to carry the ABN, because the ABN is what lets the customer claim a GST credit and what stops withholding at the top rate. This is the stamp most Australian businesses actually need.
On a company seal or a formal company mark: the ACN. The seal identifies the company as an entity, and the ACN is the entity's number. A seal carrying a tax number reads as a mismatch to anybody who knows the difference.
On a general office stamp: the ABN, usually with the trading name and address, because that stamp lands on quotes, delivery dockets and correspondence where the tax identity is what people want.
Putting both on one mark is not wrong so much as wasteful: it spends two lines on numbers where one would do, and on a 40mm round mark those lines are the whole budget.
What a tax invoice has to show
The ABN is only one item on the ATO's list. A tax invoice for a sale of 82.50 dollars or more including GST must show:
- That the document is intended to be a tax invoice.
- The seller's identity and the seller's ABN.
- The date the invoice was issued.
- A description of the items sold, including quantity and price.
- The GST amount, or a statement that the total includes GST.
- The buyer's identity or ABN, once the sale is 1,000 dollars or more.
A stamp can carry the seller's identity and the ABN, because those never change. It cannot carry the date, the amounts or the buyer, so a stamp is a supplement to an invoice rather than a way of making one.
Sole traders and the ACN that does not exist
A sole trader has an ABN and no ACN, because there is no company. A stamp for a sole trader carries the individual's name, the registered business name if there is one, and the ABN.
The mistake to avoid here is a stamp styled to look like a company mark. A sole trader mark that reads like a Pty Ltd misrepresents the entity, which is the one thing a business stamp exists not to do. See company seals: are they still required for where the line sits.
Layout and size
The rectangular office stamp is the Australian workhorse: around 56 by 22mm, three or four lines, name and ABN and address. The round seal is 38 to 40mm and carries the company name in the ring with the ACN in the centre.
Keep the number legible above everything else. It is the line somebody looks up on ABN Lookup or the ASIC register, and it is the line most often set too small. What size a stamp should be has the measurements.
For invoices arriving by email rather than on paper, the PDF stamping tool applies the mark in the browser, and the incoming side of the process is covered in invoice approval and coding stamps.
Common questions about ABN and ACN stamps
Should my company stamp show the ABN or the ACN?
The ABN if the stamp goes on invoices and everyday paperwork, because tax invoices must carry the ABN. The ACN if the mark is a company seal identifying the entity itself. Most businesses need the first far more often than the second.
Does an ABN have to be on an invoice in Australia?
A tax invoice must show the seller's ABN. Without it the customer cannot claim a GST credit, and the payer may be required to withhold at the top rate, so leaving it off costs somebody money.
Is the ABN the same as the ACN?
No, though they are related. The ACN is nine digits from ASIC and identifies the company. The ABN is eleven digits from the ATO and identifies the business for tax. Where a company has both, the ABN is usually the ACN with two extra digits in front.
Does a sole trader need a company stamp?
No, and there is no ACN to put on one. A sole trader who wants a stamp uses their own name, any registered business name, and the ABN. It should not be styled to look like a company mark.
Can I put both numbers on the same stamp?
You can, and on a rectangular stamp with room it is harmless. On a 40mm round seal it costs you two of the few lines available, so pick the one that matches what the mark is for.
The honest limits
Estamplab is a design tool, and this page describes common commercial practice rather than giving legal, tax or accounting advice. Tax invoice requirements are set by the ATO and company obligations by ASIC, and both change, so check the requirement with them or with your accountant.
Only produce a mark your own business is entitled to use. Making one for a business you do not act for is forgery.
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