BAS

How Is BAS Calculated in Australia?

Learn the BAS formula, mistakes to avoid, and why small businesses use a registered BAS agent like Simplify Bookkeeping & BAS.

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If your business is registered for GST, you’ll need to lodge a Business Activity Statement (BAS). But how is BAS calculated in Australia?

At its core, it’s about reporting the GST you’ve collected against the GST you’ve paid, plus any PAYG obligations if you have staff.

It’s one of those jobs that sneaks up every quarter. And if you’re not on top of it, the ATO can charge penalties and interest — making life stressful.

But once you understand how BAS is calculated, it’s a whole lot easier to manage.

Here is a breakdown of how to calculate your BAS, and what happens if you make an error.

What is a Business Activity Statement (BAS)?

Your Business Activity Statement is the form you use to report and pay your tax obligations to the ATO. Depending on your business, that might include:

GST (Goods and Services Tax)
PAYG withholding (tax withheld from employee wages)
PAYG instalments (your own income tax, paid in advance)
Other taxes like Fringe Benefits Tax (FBT) or Fuel Tax Credits, if they apply

Most small businesses will only need to worry about GST and PAYG.

BAS Due Dates in Australia for Small Businesses

You’ll need to lodge if:

Your business is registered for GST
You employ staff and withhold tax from their wages
The ATO sends you a BAS notice

Most small businesses will need to lodge quarterly. Larger businesses with a turnover of $20 million or more may need to lodge monthly. Some very small businesses can lodge annually if the ATO allows it.

Here are the key quarterly BAS due dates for small businesses:

A few things to keep in mind:

If your due date falls on a weekend or public holiday, it automatically shifts to the next business day.
If you lodge through a BAS agent, you’ll often get extra time to lodge and pay — one of the biggest benefits of using an agent.
Missing a deadline can mean penalties and interest charges, so it’s worth putting reminders in your calendar or outsourcing lodgement to stay on track.

How to Calculate BAS in Australia (Step by Step)

At its core, BAS is about comparing the GST you’ve collected from customers with the GST you’ve paid on your business expenses.

When you sell a product or service that includes GST, you’re collecting that 10% on behalf of the ATO. That amount doesn’t actually belong to you; it needs to be passed on.

On the flipside, when you buy goods or services for your business and pay GST, you can usually claim those amounts back as credits.

The ATO wants you to report both figures each period.

Here’s how to calculate BAS:

GST Collected on Sales (Label 1A) – GST Paid on Purchases (Label 1B) = Net GST Payable or Refund

If the result is positive, you’ve collected more GST than you’ve paid, so you owe that amount to the ATO.
If the result is negative, you’ve paid more GST than you collected, and you’ll get a refund.

This calculation is the most important part of your Business Activity Statement, and the section that trips up many small business owners if their bookkeeping isn’t accurate.

BAS Calculation Example

Let’s say in one quarter:

You’ve invoiced $33,000, including GST → GST collected = $3,000
You’ve paid $16,500, including GST on business expenses → GST paid = $1,500

Your net GST is $3,000 – $1,500 = $1,500 payable to the ATO.

If it were the other way around (more GST paid than collected), you’d get money back.

Common BAS Mistakes to Avoid

Claiming GST without a valid tax invoice
Mixing personal and business expenses
Forgetting to include PAYG withholding if you have employees
Missing deadlines (penalties start at $330 for small businesses)

A clean set of books makes BAS prep much easier and keeps the ATO off your back.

How to Lodge Your BAS

Online through the ATO Business Portal or myGov
Through your accounting software (like Xero or QuickBooks)
Via your registered BAS agent

Once lodged, you’ll either pay the ATO or wait for a refund.

Or, you can contact Simplify Bookkeeping and BAS to get a quote for us to handle BAS Lodgements on your behalf, saving you the headache.

Need help with BAS Lodgements?

BAS is one of those admin tasks that small business owners often put off, and that’s when mistakes and penalties creep in.

At Simplify, we take the stress out of bookkeeping and BAS lodgements. Our affordable services keep your numbers in order, so you can focus on growing your business without worrying about BAS deadlines.

Get in touch with Simplify today, and let’s make BAS and bookkeeping simple.

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