How to Calculate GST in Australia: Formulas, the 1/11 Rule & Quarterly BAS

The quick answer: GST is 1/11 of a GST-inclusive price

To find the GST inside a GST-inclusive price, divide the total by 11. A GST-inclusive price is made up of 11 equal parts — 10 parts are the base price and 1 part is the GST — so dividing by 11 isolates the GST. For example, $110 ÷ 11 = $10 GST (leaving a $100 base price).

If you're a sole trader, tradie or small business registered for GST in Australia, you'll calculate GST constantly — on every invoice you issue and every receipt you claim back. This guide covers the two core formulas, how to work out your quarterly GST for your BAS, and the due dates you need to know.

What is GST, and how much is it in Australia?

GST stands for Goods and Services Tax. In Australia it's a flat 10% added to the price of most goods and services. Businesses collect GST on their sales, claim back the GST on their purchases, and pay the difference to the Australian Taxation Office (ATO).

It isn't always shown clearly — sometimes GST is already included in the total (GST-inclusive), and sometimes it's added on top (GST-exclusive). Knowing which one you're looking at is the whole game.

The two core GST formulas

1. Add GST to a price (GST-exclusive to inclusive)

Multiply the base price by 1.1.

Price including GST = Base Price × 1.1  →  e.g. $100 × 1.1 = $110

2. Find the GST inside a price (GST-inclusive)

Divide the total by 11 to get the GST component, or divide by 1.1 to get the base price.

GST = Total ÷ 11  and  Base Price = Total ÷ 1.1  →  e.g. $110 ÷ 11 = $10 GST, $110 ÷ 1.1 = $100 base

How to calculate GST quarterly for your BAS

If you're registered for GST, you report and pay it to the ATO — most commonly every quarter — through your Business Activity Statement (BAS). Quarterly GST is simply the GST you collected on sales minus the GST you paid on eligible business expenses.

Net GST = GST collected − GST paid

GST collected (sales)	$4,000
GST paid (expenses)	$2,500
Net GST payable	$1,500

In this example you'd pay $1,500 to the ATO for the quarter. If you'd paid more GST than you collected — say $3,000 paid against $2,500 collected — you'd instead receive a $500 refund.

The quarterly GST periods and due dates

Quarters follow the Australian financial year. For self-lodgers, BAS is due on the 28th of the month after the quarter ends — with one exception: the December quarter gets an extra month over the holidays.

Quarter	Period	BAS due (self-lodged)
Q1	July – September	28 October
Q2	October – December	28 February
Q3	January – March	28 April
Q4	April – June	28 July

If you lodge through a registered tax or BAS agent, later dates usually apply (except for the December quarter). Always confirm the current dates with the ATO or your agent.

How to work out the GST in a total amount

Say you have an invoice total of $2,200 and need to split out the GST:

• Step 1: $2,200 ÷ 1.1 = $2,000 (base price)
• Step 2: $2,200 − $2,000 = $200 (GST)

The shortcut: $2,200 ÷ 11 = $200 GST — same answer in one step.

Stop calculating GST by hand

Doing this manually across a quarter of receipts is where mistakes (and missed credits) creep in. Instant Receipts detects the GST on every receipt, invoice and bank statement automatically, sorts each one into official ATO tax categories, and keeps a running total — so your GST figure is BAS-ready when you need it, not something you reconstruct from a shoebox at 11pm before the deadline.

A few habits that keep quarterly GST painless:

• Capture as you go: snap receipts and forward email invoices so nothing's missing at BAS time.
• Check your GST codes: make sure GST is only applied to taxable items.
• Keep your tax invoices: you need them to claim GST credits on expenses.
• Lodge on time: late BAS lodgements can attract ATO penalties.

Need to pull your numbers together? See our step-by-step guide on how to generate Tax & Compliance Reports in Instant Receipts.

What items are GST-free in Australia?

Not everything attracts GST. Common GST-free items include most fresh food, education courses, and many medical services. If GST isn't showing on something, it's likely because it's GST-free.

Frequently asked questions

What is the GST rate in Australia?

GST is 10%, applied to most goods and services. Some items — like most fresh food, medical services and education — are GST-free.

How do I calculate GST from a total amount?

Divide the total by 11. If the total is $110, the GST is $10 and the base price is $100.

How do quarterly GST payments work?

You declare the GST you collected on sales and subtract the GST you paid on expenses on your quarterly BAS, then pay the balance to the ATO (or receive a refund if you paid more than you collected).

When is my quarterly BAS due?

For self-lodgers: 28 October, 28 February, 28 April and 28 July. The December quarter's later date (28 February) reflects a built-in extension over the holiday period.

Who needs to register for GST in Australia?

You must register if your business turnover is $75,000 or more a year ($150,000 for not-for-profits). Taxi and rideshare drivers must register regardless of turnover.

Final thoughts

Calculating GST comes down to two moves: multiply by 1.1 to add it, and divide by 11 to find it inside a price. The bigger job is staying on top of GST across a whole quarter so your BAS is accurate and on time — that's where letting an app track GST automatically saves you the late-night spreadsheet and the risk of a Failure to Lodge penalty.