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Reverse GST Calculator Australia
Got a tax-invoice total and need the GST-free subtotal? Type the total and the ex-GST amount, the 10% GST component and the gross figure appear instantly — with the exact formula shown so you can check it line by line.
1 · Your figure
Ex-GST subtotal
$100.00
- GST (10%)
- $10.00
- Total incl. GST
- $110.00
$110.00 ÷ 1.10 = $100.00 ex-GST · $10.00 GST
2 · Rate & rounding
Quick reference table
Five nearby round amounts, at your rate
| Total (incl.) | Ex-GST | GST |
|---|
Rotate to landscape or widen the window for the GST column.
How the reverse GST maths works
The GST-inclusive total already contains the tax, so you divide it out instead of subtracting 10% (subtracting 10% from a GST-inclusive figure is the classic one-dollar mistake: $110 − 10% = $99, not $100).
Ex-GST = total ÷ (1 + rate ÷ 100). At 10% that is total ÷ 1.10.
GST = total − ex-GST. The two lines always add back to the amount you typed.
At the AU standard 10%, GST = total ÷ 11 — $110 ÷ 11 = $10.00, $100 ÷ 11 = $9.09.
Built for BAS and invoicing work
The headline number is the GST-free subtotal, formatted in AUD with two decimals — copy it into Xero, MYOB or your BAS worksheet.
Stay on exact cents, or round down / up to whole dollars when you reconcile a bank feed that lands on round figures.
The whole calculator is plain JavaScript in the page: no upload, no account, no request per keystroke, so invoice totals stay private.
Switch to 15% (NZ), 12.5% or 5% in one tap, or type any rate between 0% and 50%.
The division (or multiplication) is printed under the result so you can paste the working into a client email.
Numeric keypad, big tap targets and a sticky summary bar keep the answer visible while you type a receipt total.
Reverse GST FAQ
What is the reverse GST formula in Australia?
Ex-GST = GST-inclusive amount ÷ 1.10, and the GST component is the difference between the two. For a $110 invoice the GST-free subtotal is $100.00 and the GST is $10.00.
Why is removing 10% not the same as dividing by 1.1?
Because the 10% was calculated on the smaller, GST-free amount. Taking 10% off the inclusive total gives $99.00 for a $110 invoice — a dollar short. Dividing by 1.10 is the correct reverse calculation.
Is GST always 10% in Australia?
Yes — the GST rate has been 10% since it started in July 2000. Some supplies are GST-free or input-taxed, so if your invoice mixes them, reverse the GST only on the taxable lines.
Do I use the ex-GST or the GST figure on my BAS?
Both, on different labels: total sales including GST go to G1, the GST collected goes to 1A. This tool gives you each line separately so you can transfer them without re-doing the arithmetic.
Why can my figure differ by a cent?
Rounding. Accounting systems round the GST line and let the subtotal absorb the remainder, or the other way round. Pick the rounding option that matches your software and the totals will line up.
Does it handle adding GST as well?
Yes. Switch the direction to “Add GST” and the calculator multiplies your ex-GST amount by the rate and shows the inclusive total — handy when you are writing the quote rather than reading it.