Is an ACN the Last 9 Digits of an ABN?

If you’re setting up (or running) a business in Australia, you’ve probably seen the terms ABN and ACN come up everywhere - on invoices, contracts, websites, and government forms.

And if you’ve ever looked at an ABN and thought, “Hang on, isn’t the ACN just the last 9 digits of the ABN?” you’re not alone. It’s a very common question for business owners, especially when you’re registering a company or checking details for a customer or supplier.

In this guide, we’ll walk you through what an ABN and ACN actually are, when they overlap, when they don’t, and what you should be putting on your business documents so you stay compliant (and avoid confusing your customers, suppliers, or lenders).

Is The ACN The Last 9 Digits Of The ABN?

Often, yes - for companies, the ACN is usually the last 9 digits of the ABN.

Here’s the important detail that trips people up:

• An ACN (Australian Company Number) is a 9-digit number issued to companies by ASIC.
• An ABN (Australian Business Number) is an 11-digit number issued to a business entity (which can be a company, sole trader, partnership, trust, or incorporated association, depending on the setup).

If you operate through a company, your company will have both an ACN and an ABN. In that situation, your ABN is typically structured like this:

• ABN = 2-digit prefix + your 9-digit ACN

So if you’re wondering whether the ACN is the last 9 digits of an ABN, the practical answer is:

Yes - if the entity is a company, the ACN will generally match the last 9 digits of the ABN.

But that doesn’t mean every business has an ACN (because not every business is a company), and it doesn’t mean you can always assume an ABN “contains” an ACN.

What’s The Difference Between An ABN And An ACN?

The simplest way to think about it is:

• ABN identifies a business entity for tax and trading purposes.
• ACN identifies a company registered with ASIC under the Corporations Act.

What Is An ABN?

An ABN is an 11-digit identifier used in Australia for business dealings, invoicing, GST, and interacting with government agencies.

You can have an ABN if you operate as:

• a sole trader
• a partnership
• a company
• a trust (with a corporate trustee or individual trustee)
• some incorporated associations and other entity types

ABNs are used widely in day-to-day business, including when you:

• issue invoices
• register for GST
• set up business accounts with suppliers
• get paid by customers (especially other businesses)

It’s also worth knowing that ABN details are public and are commonly checked as part of basic business due diligence (for example, making sure the entity you’re paying is legitimate).

What Is An ACN?

An ACN is a unique 9-digit number ASIC gives to a company when it’s registered. You’ll only have an ACN if your business structure is a company.

This includes:

• proprietary limited companies (Pty Ltd)
• public companies
• special purpose companies (in relevant contexts)

Because an ACN is tied to ASIC registration, it’s a key “identity marker” for company compliance, including how your company is described on legal documents, invoices, and correspondence.

When Will Your Business Have Both An ABN And An ACN?

Your business will have both if you’re operating through a company and you’ve registered for an ABN in the company’s name.

This is extremely common for small businesses because many business owners choose a company structure for:

• limited liability (this can help reduce personal exposure for company debts in many situations, though directors can still be personally liable in some circumstances)
• a clearer structure for bringing in co-founders or investors
• more credibility for some customers, suppliers, and lenders
• potential tax and planning outcomes (which depend on your circumstances - it’s best to speak with an accountant or tax adviser)

If you’re currently deciding whether to run your business through a company, it’s usually worth getting your “foundation documents” sorted at the same time - for example a Company Constitution that matches how you actually want the business to operate.

And if you’re starting with other founders, a Shareholders Agreement can be one of the most important documents for preventing disputes later (especially around decision-making, exits, and what happens if someone stops working in the business).

When You Won’t Have An ACN (Even If You Have An ABN)

If you’re not a company, you won’t have an ACN - even though you can still have an ABN.

Common examples include:

• Sole trader: you can have an ABN, but you will not have an ACN.
• Partnership: the partnership can have an ABN, but no ACN (unless a partner is a company and you’re looking at that partner’s details separately).
• Trust: the trust may have an ABN, and the trustee might be a company (meaning the trustee has an ACN), but the ABN doesn’t automatically “contain” a trust ACN because trusts themselves do not get ACNs.

So if you’re looking at an ABN and can’t find the ACN in it, it may simply be because there isn’t one.

How Do You Check If The ACN Matches The ABN?

If you’re dealing with a company and you want to confirm whether the ACN is the last 9 digits of the ABN, you can do a quick sanity check:

1. Take the 11-digit ABN.
2. Ignore the first 2 digits.
3. The remaining 9 digits should match the company’s ACN.

That said, in real life, you’ll also want to make sure you’re looking at the right entity. Many business groups have multiple entities (for example, a holding company and an operating company, or a trustee company for a trust).

If you’re entering into a contract, issuing credit, or signing a long-term agreement, it’s smart to confirm you have the correct legal entity details upfront. This is one of those “small admin steps” that can save you major headaches if something goes wrong later.

ABN Vs Business Name: Don’t Mix Them Up

Another common confusion is between:

• the ABN (the identifier of the legal entity), and
• the business name (the trading name that appears on branding, signage, and websites).

Your business name doesn’t replace your ABN or ACN - it’s more like a public-facing label for your business. If you’re unsure how these pieces fit together, entity name vs business name is a helpful way to think about it, especially when you’re signing contracts or setting up invoices.

What Number Do You Need To Put On Invoices, Websites, And Contracts?

Once you understand the relationship between ABNs and ACNs, the next question is usually: “What do I actually need to show customers and suppliers?”

There are a few common places this comes up.

Invoices

For most small businesses, your invoices should clearly show the details of the entity that is charging for the goods or services, including the correct ABN.

If you’re a company, your ACN may also need to be shown in certain contexts - particularly if you’re not using your company name in full (for example, if you’re trading under a business name). As a practical matter, including both the ABN and ACN on invoices can improve clarity, as long as it’s accurate.

If your invoice terms and payment expectations aren’t documented anywhere (or are copied from a generic template), it can be worth formalising them in Terms of Trade, especially if you offer credit, charge late fees, or want clearer dispute processes.

Websites And Online Stores

If you run a website (including an eCommerce store), it’s common to include your business details in the footer, your contact page, and your legal pages.

Most businesses should also have a Privacy Policy if they collect personal information (for example, customer names, emails, delivery addresses, or analytics identifiers). While this is separate to your ABN/ACN question, it’s part of building a compliant online presence and avoids awkward gaps in your legal setup.

Contracts And Client Agreements

Where ABN/ACN mistakes cause the biggest problems is usually contracts.

If your customer contract, supplier agreement, or service agreement lists the wrong entity, you can end up with:

• uncertainty about who is actually responsible for delivering the goods or services
• difficulties enforcing payment or late fees
• disputes about who owns intellectual property created under the contract
• issues if you later sell the business, restructure, or bring in investors

If you provide services to clients, a properly drafted Service Agreement is a practical way to make sure the right entity is contracting, and that your scope, fees, limitations, and risk settings are clear from day one.

Common Mistakes Small Businesses Make With ABNs And ACNs (And How To Avoid Them)

Even though ABNs and ACNs are “admin details”, they can create real legal and commercial risk when they’re wrong or inconsistent. Here are some of the most common traps we see, and what you can do to avoid them.

1. Using Your Business Name As If It’s The Legal Entity

Your business name is not the legal entity. If you sign a contract as “Coastal Creative Studio” but the legal entity is actually “Coastal Creative Studio Pty Ltd” (or an individual sole trader), you’re creating confusion before the relationship even starts.

A good habit is to ensure your key documents always show:

• the legal entity name (company name, individual name, trustee name, etc.)
• the ABN (and ACN if relevant)
• the business name (if you trade under one), clearly labelled as the trading name

2. Not Updating Details After A Restructure

As your business grows, you might restructure - for example:

• moving from sole trader to company
• introducing a trust for asset protection or tax planning
• setting up a new entity for a new venture

When this happens, your ABN and ACN details may change depending on the new structure. Your contracts, invoices, website footer, and terms should be reviewed so they match the entity actually operating the business.

3. Assuming Every ABN Has An ACN Inside It

This is the core issue behind the question of whether an ACN is the last 9 digits of an ABN. It’s often true for companies, but it’s not true for every entity with an ABN.

If you’re doing due diligence on another business and you can’t find an ACN, ask:

• Are they a company?
• Are they a sole trader or partnership?
• Are you dealing with a trust, and the trustee is the company?

4. Forgetting You May Need More Than A Number To Protect Your Business

ABNs and ACNs are identifiers - they don’t, by themselves, protect your brand, your relationships, or your revenue.

If you’re building a business you want to grow, your legal setup often needs to include:

• clear customer or client terms
• proper contractor or employment arrangements
• privacy and data compliance
• founder/investor documents if you’re not doing it alone

For example, if you’re hiring, the right Employment Contract can help set expectations around duties, confidentiality, and notice periods, and reduce the risk of disputes down the line.

Key Takeaways

• For companies, the ACN will generally match the last 9 digits of the ABN - the ABN is usually a 2-digit prefix plus the 9-digit ACN.
• Not every business has an ACN; only companies registered with ASIC have an ACN, but many other entity types can still have an ABN.
• ABNs identify a business entity for trading and tax purposes, while ACNs identify a company for ASIC registration and corporate compliance.
• Getting your entity details right matters on invoices, websites, and especially contracts - using the wrong entity can create enforceability and payment issues.
• Business names are not the same as entity names, so make sure your documents clearly show the correct legal entity and its ABN (and ACN if relevant).
• As your business grows, your legal documents should keep up - especially if you restructure, bring on co-founders, or start signing larger contracts.

Note: This article is general information only and isn’t legal or tax advice. For guidance on ABNs, GST, and tax obligations in your specific circumstances, consider speaking with your accountant or a registered tax agent, or checking the ATO’s resources.

If you’d like help getting your business structure and documents set up correctly (including making sure you’re using the right ABN/ACN details in your contracts), you can reach us at 1800 730 617 or team@sprintlaw.com.au for a free, no-obligations chat.