Calculating GST on an invoice shouldn't take a calculator and a finance degree. If you're an Australian tradie registered for GST, here are the two formulas you'll use every day — plus worked examples so you can check your invoices are right every time.
How to calculate GST on an invoice — the two formulas
GST in Australia is always 10%. That means there are only two situations you'll ever face:
1. Adding GST to a price (GST-exclusive → GST-inclusive)
Use this when you know your pre-GST price and need to work out how much to charge the customer.
Formula: GST amount = Price × 0.10
Total to charge: Price + GST = Price × 1.10
Worked example: You complete an electrical job and your labour and materials come to $850 (before GST).
- GST = $850 × 0.10 = $85.00
- Total invoice amount = $850 + $85 = $935.00
2. Finding GST in a GST-inclusive price (working backwards)
Use this when you've already quoted a total price and need to show the GST component separately on your ATO-compliant Tax Invoice.
Formula: GST amount = Total price ÷ 11
Pre-GST price: Total price − GST = Total price × (10 ÷ 11)
Worked example: You quoted a plumbing job at $1,320 all-inclusive. The customer needs the GST shown separately on their invoice.
- GST = $1,320 ÷ 11 = $120.00
- Pre-GST price = $1,320 − $120 = $1,200.00
Memory trick: Adding GST? Multiply by 1.10. Finding GST in a price? Divide by 11. That's all you need to remember.
How to show GST on an invoice correctly
The ATO requires the GST amount to be shown separately on every Tax Invoice. A correct invoice shows three lines in the totals section:
| Line | Example amount |
|---|---|
| Subtotal (excl. GST) | $850.00 |
| GST (10%) | $85.00 |
| Total (incl. GST) | $935.00 |
Writing "GST included" in the description is not enough — the dollar amount must be stated. A free invoice app for tradies like TradesBill handles this automatically, so you never need to do the maths manually or worry about formatting it correctly.
What if some items on my invoice don't have GST?
Some items are GST-free (like certain food, medical services, and some exported goods) or input-taxed. As a tradie, almost everything you charge — labour, materials, call-out fees — will be taxable at 10%. But if you have mixed items:
- List taxable and GST-free items separately on the invoice
- Apply 10% only to the taxable items
- Show the total GST as a single line at the bottom
- Clearly label which items are GST-free if any
When in doubt, check with your accountant or BAS agent — they can confirm which of your services attract GST.
How to calculate GST for your BAS
When you complete your Business Activity Statement (BAS), you report the total GST you collected from customers and the total GST credits you can claim on your business purchases. The net amount is what you pay to (or receive back from) the ATO.
- GST collected — add up the GST amounts from all Tax Invoices you issued during the period
- GST credits — add up the GST on all your eligible business purchases (materials, tools, fuel, etc.)
- Net GST = GST collected − GST credits
If your GST collected is more than your credits, you pay the difference to the ATO. If your credits are more, the ATO refunds you.
TradesBill Pro includes BAS-ready Tax Summary Reports — a breakdown of your GST collected per quarter, ready to hand to your BAS agent or enter directly into your myGov account. Try Pro free for 14 days.
Do I need to register for GST?
You must register for GST if your annual turnover is $75,000 or more. Once registered, you must add GST to all your taxable sales and issue proper ATO-compliant Tax Invoices. If you're under the threshold, GST registration is optional — but you can't charge GST or claim GST credits if you're not registered.
Let TradesBill calculate GST for you
Add your line items and TradesBill automatically calculates the subtotal, GST (10%), and total — and puts them on a properly formatted Tax Invoice. Free forever.
Start Free — No credit cardFrequently asked questions
Is GST always 10% in Australia?
Yes. The GST rate in Australia has been 10% since it was introduced on 1 July 2000 and has not changed.
What if I round the GST amount?
The ATO allows GST amounts to be rounded to the nearest cent. So $85.455 rounds to $85.46. Most invoicing apps handle this automatically.
Can I charge GST if I'm not registered?
No. If you're not registered for GST, you cannot charge GST and cannot issue a Tax Invoice. Charging GST without being registered is a compliance issue. If your turnover is approaching $75,000, register before you cross the threshold.
Does GST apply to quotes?
Your quote should state whether prices are GST-inclusive or GST-exclusive. The most common approach for tradies is to quote GST-exclusive and then add "plus GST" — that way the customer sees both the base price and what they'll actually pay.