Reverse GST Calculator Australia: Strip 10% GST in Seconds
Reviewed by the FreeOnline.fyi team · Updated 2026-09-16
Why reverse GST matters at BAS time
Most receipts a sole trader ends up holding show exactly one number: the total paid, with GST already baked into it. A BAS or a bookkeeping entry wants something different — the GST-free portion of that supply and the GST component you're claiming as a credit, kept apart so each can go in its own box. That means the single figure on the docket has to be pulled back into two before it's any use in Xero, MYOB or a spreadsheet.
At the standard Australian rate, the GST hidden inside a total is one eleventh of it, not one tenth. The ATO's guidance on how GST works is the authority here, and the arithmetic follows from it: divide a GST-inclusive amount by 1.1 and the remainder is the GST.
We built the Reverse GST Calculator Australia so that split happens while you're still holding the invoice. Type the total, read the two numbers, move on.
Typing the total and reading the split
The page opens with the amount field already focused and a default value of 110 sitting in it, which is a deliberately useful starting point: 110 inclusive becomes 100 ex-GST plus 10 GST, so the shape of the answer is visible before you type anything. Replace it with your own figure and the result card on the right updates with every keystroke — no button to press, because a single-number calculation shouldn't need one.
The mode toggle defaults to Remove GST (inclusive to exclusive), which is the direction you want when working backwards from a tax invoice. Flip it to Add GST when the number you hold is already ex-GST and you need the total to quote or invoice: 850 ex-GST becomes 935 gross with an 85 GST line. The result labels swap with the mode, so the big figure is always the one you asked for.
Underneath, a formula line spells out the exact operation performed — for example "110.00 ÷ 1.10 = 100.00 ex-GST · 10.00 GST" — which is worth reading the first few times so you can reproduce the maths in a spreadsheet later. On a phone the card stacks directly beneath the amount field and a compact summary bar sticks to the screen as you scroll down to the table.
Divide by 1.1, never multiply by 0.9
The classic error is treating the 10% as something you can subtract with a single multiplication. Take 110 inclusive and multiply by 0.9 and you get 99 ex-GST with 11 GST. The correct answer is 100 and 10, because the 10% was charged on the ex-GST amount, not on the total. You've just overstated your GST credit by a dollar.
Scale that up and it starts costing real money. On an 11,000 invoice the 0.9 method produces 9,900 ex-GST and 1,100 GST, when the true split is 10,000 and 1,000 — a hundred dollars of phantom credit sitting in your BAS, waiting to be corrected in an audit or a reconciliation.
The general formula the calculator uses is ex-GST = total ÷ (1 + rate ÷ 100), then GST = total − ex-GST. It behaves correctly for other rates too: switch to the 15% chip and 115 inclusive becomes 100 plus 15, and 12.5% or 5% work the same way. Keep the rate at 10 unless you're deliberately modelling a different jurisdiction — Australian GST has been 10% since 2000, and the other chips exist for New Zealand and for what-if checks, not for Australian supplies.
Rounding rules and the quick table
By default every figure is rounded to the nearest cent, which is what you want for bookkeeping and what matches the amounts your accounting software will store. Two alternatives are offered — round down to whole dollars and round up to whole dollars — for the situations where cash handling or a tidy mental model takes priority over cent precision. If you use a whole-dollar setting, apply it consistently across the month rather than switching mid-period, because a pattern of always rounding in your favour is the kind of thing a reconciliation will surface.
One detail worth knowing about how the rounding works here: the ex-GST amount is calculated first and the GST component is derived by subtracting it from the gross. The three displayed numbers therefore always agree with each other — ex-GST plus GST equals the total to the cent — instead of drifting apart the way independent rounding of two figures can.
The quick table below the result lists five nearby round amounts with their ex-GST and GST columns side by side, which turns out to be the fastest part of the tool when you're processing a stack of similar receipts. Seeing at a glance that 220 inclusive holds 20 of GST, 330 holds 30 and 550 holds 50 lets you sanity-check the calculator's answer on your own invoice before you commit it to the ledger.
When the 10% split doesn't apply
Dividing by 1.1 is only correct when the entire amount is a taxable supply at 10%. Plenty of real invoices aren't. Basic food, many medical and health services, education courses and exports can be GST-free, and financial services and residential rent are input-taxed — in both cases there's no GST embedded in that line at all, so a reverse calculation applied to the total will invent a tax credit that doesn't exist.
Mixed invoices are the common trap. If a 110 total contains 55 of GST-free items, the GST is not 10. The correct approach is to separate the taxable lines from the non-taxable ones, reverse the GST out of the taxable portion only, and add the rest back in untaxed. The calculator is a two-number machine; it can't know what's inside your total.
There are also scheme-specific cases — the margin scheme on some property and second-hand goods, GST groups, and supplies to overseas recipients — where the arithmetic is not a simple rate applied to a total. The business.gov.au guidance on GST is a reasonable starting point. If your situation involves any of these, treat the calculator as a quick check and confirm the treatment with your accountant; it performs arithmetic, it doesn't give tax advice.
From receipt back to bookkeeping
The practical loop is short. Pull the total off the receipt, put it in the amount field, read the ex-GST figure into the expense line and the GST figure into the tax credit, and note the supplier and date as usual so the invoice can be matched later. Because the formula line stays visible, you can copy the exact division into a spreadsheet formula if you'd rather batch a month's worth of receipts at once.
The same ex-GST number matters on the revenue side. Turnover reported to the ATO is GST-exclusive, and it's also the base that earnings multiples get applied to when someone is sizing up a business — the Online Business Valuation Tool works from figures of that kind, so an inflated gross revenue number carried into a valuation will skew the result.
Everything here runs in the browser and the amount you type never leaves the page, which is the right default for financial figures. There's a wider set of free online tools on FreeOnline.fyi if you need a calculator for something adjacent. Use the numbers as a fast, reliable check, but verify anything that ends up on a lodged BAS against the source documents.