Australia's Goods and Services Tax is a flat 10% added at every point most goods and services change hands, and unlike US sales tax it doesn't vary by state, postcode, or product category the way you might expect from a country with eight separate state and territory governments. For freelancers and small businesses invoicing Australian clients — or checking what a supplier's GST-inclusive quote actually breaks down to — the only real complexity is remembering that most listed prices already include the tax, so "removing" GST is usually the calculation you need, not "adding" it.
How it works
Remove GST (the default here, because AU retail and consumer prices are legally required to be GST-inclusive) takes a total that already has GST baked in and works backwards. Since a GST-inclusive total equals the pre-GST price times 1.10, the GST component of that total is exactly one-eleventh of it — not 10% of it, which would overstate the tax. This calculator divides the amount by 11 to get the GST, then subtracts that from the amount to get the price excluding GST.
Add GST runs the more familiar direction: you supply a GST-exclusive price — a wholesale cost, a B2B quote, a rate you're building up from scratch — and the calculator adds 10% on top to produce the GST-inclusive total a customer would actually pay.
Both modes round every dollar figure to the cent from the unrounded math, so the GST amount and the total or ex-GST amount always reconcile against each other to the cent, regardless of which direction you're solving.
Worked example
Someone hands you a $1,100 GST-inclusive invoice and you need to know how much of that was GST, for your own books or a BAS lodgment. In remove mode:
- GST amount: $1,100 ÷ 11 = $100.00
- Amount excluding GST: $1,100 − $100 = $1,000.00
Now flip it: you're quoting a client a $345.67 GST-exclusive rate and need the total to invoice them, in add mode:
- GST amount: $345.67 × 10% = $34.57
- Total (GST-inclusive): $345.67 + $34.57 = $380.24
Notice that in the first example, 10% of $1,100 would have been $110 — thirty cents higher than the correct $100 GST amount. That gap is exactly why remove mode divides by 11 instead of taking 10% of the inclusive figure directly, and it's the single most common manual GST mistake this tool exists to avoid.
How to interpret your result
Which output matters depends on what you're doing. Quoting or invoicing a client from a GST-exclusive base: use add mode, and the total is what you charge and what appears on the tax invoice. Reconciling a receipt, extracting the GST component from a supplier's GST-inclusive quote, or filling in a Business Activity Statement (BAS) from a GST-inclusive figure: use remove mode, and the GST amount is what you report as GST collected or paid, while the ex-GST amount is your real revenue or cost figure — not the total.
If you're a freelancer or small business deciding whether you even need to charge GST at all, that depends on your registration status, which this calculator doesn't know. Businesses only charge and remit GST once they're registered, which is compulsory once GST turnover reaches (or is expected to reach) the threshold covered below — below that, plenty of sole traders simply don't charge GST and this tool isn't relevant to their invoices at all.
Methodology & sources
Add mode: gstAmount = round2(amount × 0.10), total = round2(amount + gstAmount). Remove mode: gstAmount = round2(amount ÷ 11), exGstAmount = round2(amount − gstAmount), with total equal to the GST-inclusive amount you entered. Dividing by 11 in remove mode is standard GST-inclusive-to-exclusive arithmetic, not a rounding shortcut — it falls directly out of total = exGst × 1.10.
The 10% rate is current as verified against the Australian Taxation Office's How GST works and How Australian GST works pages, which describe GST as "a broad-based tax of 10% on most goods, services and other items sold or consumed in Australia." It has held at 10% since GST commenced on 1 July 2000. The $75,000 GST registration turnover threshold ($150,000 for non-profit organisations), described on the ATO's Registering for GST page, has likewise been unchanged since 2000 — but always confirm both figures directly on ato.gov.au before relying on them, since this tool performs arithmetic on numbers you supply and is not a substitute for registration advice, BAS preparation, or a registered tax agent.