Australia's GST is exactly 10% — but removing it from a total isn't as simple as dividing by 10. Here's the ATO formula and a free calculator.
Imagine a tradesperson gets paid $1,100 for a job. How much is their actual income and how much belongs to the ATO? Most people instinctively multiply $1,100 × 10% and get $110 — but that is the wrong number. The correct answer uses the ATO's 1/11 method: $1,100 ÷ 11 = $100 GST. This page covers the exact ATO formula, 3 worked examples, a free calculator, BAS context, and the full GST-free items list.
Reverse GST Calculator Australia
Enter any GST-inclusive price. Instantly see the GST-exclusive amount and the exact GST charged.
Open Free GST Calculator →No signup · ATO 1/11 method · Works on mobile
What Is Reverse GST?
GST (Goods and Services Tax) is a 10% tax added to most goods and services in Australia. When a business charges $110 for a service, $100 is their revenue and $10 is GST they are collecting on behalf of the ATO. Reverse GST calculation finds the $100 base from the $110 total.
This matters most for quarterly BAS lodgement, claiming GST credits on business purchases, and correctly splitting income from tax collected. If you issue or receive invoices, you need to know both the GST-inclusive and GST-exclusive amounts.
The ATO Formula — Why GST Is 1/11, Not 10%
Wrong: $110 × 10% = $11 ✗
Right: $110 ÷ 11 = $10 ✓
The confusion comes from how GST is structured. The 10% rate applies to the GST-EXCLUSIVE base price — not the final total. $100 × 10% = $10 GST, making the total $110. When you work backwards from $110, you are not looking for 10% of $110 ($11) — you are looking for 10/110 of $110, which simplifies to 1/11. This is the figure the ATO instructs you to use on every BAS.
The 10% Mistake
$110 × 10% = $11 GST ✗
You'd remit $1 too much to the ATO every time.
The ATO's 1/11 Method
$110 ÷ 11 = $10 GST ✓
Exact — matches what you charged on your invoice.
Worked Examples
Example 1 — Simple receipt: $110 GST-inclusive
Ex-GST price = $110 − $10 = $100.00
Example 2 — Café bill: $55.00 GST-inclusive
Ex-GST price = $55 − $5 = $50.00
Note: this only applies if the item is not GST-free (fresh food is exempt — see below).
Example 3 — Contractor invoice: $1,650 GST-inclusive
Ex-GST price = $1,650 − $150 = $1,500.00
Note: This is the amount you remit to the ATO on your next BAS. The $1,500 is your taxable income.
How to Add GST to a Price
Adding GST is straightforward: multiply the GST-exclusive price by 1.1.
$100 × 1.1 = $110.00
$500 × 1.1 = $550.00
$2,000 × 1.1 = $2,200.00
When you quote a client, be explicit whether your price is ex-GST or GST-inclusive. Adding 'plus GST' to a quote means multiply your number by 1.1 to get what they actually pay.
When Do You Need to Reverse-Calculate GST?
Lodging Your BAS (Business Activity Statement)
Every GST-registered business in Australia must lodge a BAS — usually quarterly. The BAS requires you to report total GST collected (from sales) and total GST paid (on purchases you can claim back). Both figures must be the exact GST component, not the gross total. Reverse-calculating every invoice and receipt is mandatory for an accurate BAS. Errors trigger ATO reviews.
Claiming GST Credits on Business Purchases
If you purchase supplies for your business and pay GST on them, you can claim that GST back as a credit against what you owe the ATO. To do this, you need a valid tax invoice and the exact GST amount. Reverse-calculating from the GST-inclusive total gives you the credit amount. Example: you buy $330 of office equipment — that is $30 GST you can claim, reducing your net BAS liability.
Verifying Supplier Invoices
A tax invoice must separately state the GST amount. If a supplier provides only a GST-inclusive total without itemising the GST, you can verify it yourself: total ÷ 11 = the GST that should be shown. If the numbers don't match, the invoice may not be valid for GST purposes and cannot be used to claim credits.
For a full guide to the reverse-tax formula across all countries, see how to calculate reverse tax.
GST-Free Items in Australia
Not everything in Australia attracts GST. The ATO maintains a list of GST-free supplies where businesses charge 0% GST. Knowing what's excluded matters when you're calculating: a $10 supermarket receipt for fresh vegetables contains $0 GST, not $0.91.
| Category | Examples | GST Status |
|---|---|---|
| Fresh food | Fruit, vegetables, meat, bread | GST-free (0%) |
| Medical | Doctor visits, hospital, medicines | GST-free (0%) |
| Education | School fees, TAFE, university | GST-free (0%) |
| Exports | Goods sold to overseas customers | GST-free (0%) |
| Financial services | Bank fees, insurance premiums | Input-taxed |
| Residential rent | Long-term residential leases | Input-taxed |
| Most goods/services | Electronics, clothing, trades, food at cafés | Taxable (10%) |
Frequently Asked Questions
How do I calculate GST from a total in Australia?
Use the ATO's 1/11 method: divide the GST-inclusive total by 11. For example, $110 ÷ 11 = $10 GST, leaving $100 as the ex-GST price. Do not multiply the total by 10% — that gives the wrong answer because GST is 10% of the base price, not 10% of the final total. Our free GST calculator does this instantly.
Is reverse GST always 1/11?
Yes — Australia's GST is a flat 10% rate, so the reverse calculation is always Total ÷ 11. This never changes regardless of what you're buying or which state you're in. The only exception is GST-free items (fresh food, medical, education) where no GST was charged, so there is nothing to remove.
What items are GST-free in Australia?
The main GST-free categories are: basic food (fresh fruit, vegetables, meat, bread), medical and health services, education fees, and exported goods. Financial services and residential rent are "input-taxed" — a separate category where GST doesn't apply but the credit rules differ. Most other goods and services are taxable at 10%.
How do I claim GST back in Australia?
If you are registered for GST, you can claim GST credits (Input Tax Credits) on business purchases by reporting them on your BAS. You need a valid tax invoice that shows the supplier's ABN, the GST amount, and the GST-inclusive total. The credit reduces the GST you owe to the ATO for that quarter.
What is a tax invoice for GST purposes?
A tax invoice is a specific document the ATO requires to claim GST credits. For purchases over $82.50 (GST-inclusive), it must include: the words "tax invoice", the supplier's name and ABN, the date, a description of the goods or services, the GST-inclusive price, and either the GST amount or a statement that GST is included. Receipts without an ABN or GST breakdown are not valid tax invoices.
Whether you're reconciling a BAS, checking a supplier invoice, or simply curious what a price costs before tax, our free reverse GST calculator gives you the exact ATO-method result in one click.
Sources & Citations: Content verified against official guidelines from the IRS (US), HMRC (UK), and ATO (AU). Information is reviewed for accuracy prior to publication.
Free Calculator
Reverse Tax Calculator
Find the original pre-tax price from any total. Supports US sales tax by state, UK VAT, Australia GST, Canada GST/HST, and any custom rate.
Comments are coming soon. Have feedback? Contact us.