Reverse GST Calculator (Remove GST from a Price)
Reverse GST Calculator
So here it is. Drop a GST-inclusive total into the calculator above and you’ll get the GST amount and the ex-GST price in the same click. We’ve also explained the formula below, with a worked example, the exact mistake most people make once and the situations where reversing GST actually matters.
What does ‘reverse GST’ mean?
Reverse GST is the calculation that takes a GST-inclusive total and breaks it back into its two parts: the original ex-GST price and the 10% GST. It’s the opposite of adding GST – instead of multiplying by 1.1, you divide.
The Australian Taxation Office calls these GST-inclusive amounts ‘tax-inclusive prices’, and most prices you see at the till in Australia (a Coles trolley, a Bunnings tile order, a JB Hi-Fi headphone box) are already inclusive. Reversing the GST is how you find the underlying value the supplier earned before tax.
How to calculate GST backwards manually
The maths is short. Three lines is enough.
- GST amount = Total ÷ 11
- Ex-GST price = Total ÷ 1.1
- Verification: GST amount + ex-GST price = original total
The reason we divide by 11 (and not 10) is that the GST sits on top of the ex-GST price, not on top of the total. The 10% has already been added in, so the GST is one-eleventh of what you’re holding, not one-tenth.
Worked example: a $1,100 Officeworks invoice
Say you’re a sole trader and you’ve just bought a $1,100 standing desk from Officeworks. The invoice shows the GST-inclusive total but doesn’t itemise the split.
Do the maths:
- GST = $1,100 ÷ 11 = $100
- Ex-GST price = $1,100 ÷ 1.1 = $1,000
- Check: $1,000 + $100 = $1,100 ✓
That $100 is the GST credit you can claim on your next BAS at label 1B (GST on purchases). The $1,000 is what your accountant records as the deductible business expense.
When you’d reverse-calculate GST
The calculator earns its keep in a handful of real situations:
- BAS preparation. When you’re tallying GST credits at label 1B for the quarter, every supplier invoice needs the GST broken out. If your software hasn’t done it for you, this is the formula you’re applying.
- Expense claims. Sole traders, contractors and freelancers reversing GST from receipts to record the deductible portion separately from the GST credit.
- Tax invoice spot-checks. If a supplier’s invoice doesn’t itemise GST clearly, dividing the total by 11 confirms what the GST line should be.
- Quoting backwards from a budget cap. A client says ‘I’ve got $5,500 to spend’ – you reverse-calculate to find the $5,000 ex-GST quote that fits.
- Rideshare and delivery payouts. Uber, DoorDash and Menulog statements often arrive as totals; reverse-calculating gives you the GST you’ve collected.
Reverse GST formula reference
For anyone working in Excel, Google Sheets or a quick phone calculator:
- Excel/Sheets GST amount: =A1/11
- Excel/Sheets ex-GST price: =A1/1.1
- Phone calculator: total ÷ 11 = GST; total ÷ 1.1 = ex-GST
Keep this in mind: rounding can leave you a cent short on long invoices. The ATO accepts rounding to the nearest cent, but software like Xero and MYOB rounds at the line level, which can quietly add up across a BAS quarter.
The mistake almost everyone makes once
The classic error is reaching for ÷ 10 instead of ÷ 11. It feels right because GST is 10%. But it isn’t.
If you take 10% off $1,100, you get $990. That’s wrong. The correct ex-GST figure is $1,000. The reason: 10% of $1,100 is $110, but only $100 of GST was ever charged on the original $1,000. Subtracting 10% of the inclusive total overstates the GST by $10.
Plain rule – subtract 1/11, not 10%. Or just divide by 11 and stop second-guessing yourself.
Reverse GST vs GST exclusive – the difference
These terms get mixed up constantly. Here’s the line.
A reverse GST calculator starts from a GST-inclusive total and works backwards to find the pre-GST amount. A GST exclusive price is a price that has been quoted without GST, often used in B2B contracts and wholesale dealings. The output of a reverse calculation is a GST exclusive price, but the calculation only makes sense when the input is inclusive.
If a supplier quotes you a GST exclusive price, you’re not reversing anything – you’d be adding GST. We’ve got a separate add GST calculator for that.
Related calculators
- GST Calculator (homepage) – the all-in-one add and reverse tool
- Add GST Calculator – multiply by 1.1
- GST Formula reference – every AU GST formula on one page
- How to add GST to an invoice – tax invoice rules from the ATO
FAQs
How do you calculate GST backwards in Australia?
Divide the GST-inclusive total by 11 to get the GST amount, or by 1.1 to get the ex-GST price. So a $1,100 total contains $100 of GST and a $1,000 ex-GST price.
What is the formula to remove GST from a price?
The formula is Total ÷ 1.1 = ex-GST price, and Total ÷ 11 = GST amount. Both come from the same maths because GST is 10% added to the ex-GST figure, which makes it one-eleventh of the total.
Is GST 10% or 1/11 of the total?
Both, depending on which side you’re starting from. GST is 10% of the ex-GST price (the smaller figure), and 1/11 of the GST-inclusive total (the bigger figure). They describe the same dollar amount from two angles.
How do I work out the GST on a $1,100 invoice?
$1,100 ÷ 11 = $100 GST. The ex-GST portion is $1,000. If your invoice also includes GST-free items (basic food, certain medical supplies, exports), reverse only the taxable portion.
What’s the difference between reverse GST and GST exclusive?
Reverse GST is the calculation you run on a GST-inclusive total to extract the pre-GST amount. A GST exclusive price is the result of that calculation, or a price quoted without GST in the first place. Reverse-calculating only applies when GST is already baked in.
Can I use this for BAS reporting?
Yes. The reverse GST figure is what you’d record at label 1B (GST on purchases) for any inclusive supplier invoice. For sales at label 1A, use the same divide-by-11 logic on each GST-inclusive amount you’ve billed in the quarter. The ATO publishes worked examples on its BAS guide at ato.gov.au.
Last updated: 9 May 2026 · By Bobby
The calculators and guides on gstcalculator.net.au are for general information only and do not constitute tax, financial, or legal advice. Consult a registered tax agent for advice specific to your situation.