How we work it out
Adding GST is the amount times 1.10. Removing GST from a total that already includes it is the amount divided by 1.10, not just subtracting 10%, since 10% of the GST-inclusive total isn't the same as 10% of the original price underneath it.
Worked example, adding GST: a $100 price excluding GST comes to $10 of GST and a $110 total including it.
Worked example, removing GST: a $110 total that already includes GST breaks down to $100 excluding GST and $10 of GST, not $11, which is what a flat 10% subtraction would wrongly give you.
Why you can't just subtract 10% from a GST-inclusive total
Ten percent GST is calculated on the price before GST, not on the total after it's added. So working backwards from a $110 GST-inclusive total, subtracting a flat 10% ($11) leaves $99, which is wrong. Dividing by 1.10 instead gives the correct $100 base price, with $10 of GST sitting on top of it. This trips people up constantly when pulling GST back out of a receipt or an invoice total.
When GST applies
GST is charged at a flat 10% on most goods and services sold in Australia, with some exemptions: most basic food, many medical and education services, and some exports. A business only has to charge and remit GST once it's registered, which becomes compulsory once annual turnover reaches $75,000. This calculator applies the standard 10% rate; it doesn't check whether a specific item is GST-free.
Common questions
How do you add GST to a price?
Multiply the price by 1.10. A $100 price excluding GST becomes $110 including it, with $10 as the GST component.
How do you remove GST from a total?
Divide the GST-inclusive total by 1.10, don't just subtract 10%. A $110 total including GST breaks down to $100 excluding GST and $10 of GST.
Is GST always 10% in Australia?
Yes, the standard Australian GST rate is a flat 10%, though some goods and services (like most basic food, many medical and education services) are GST-free.
Do I have to register for GST?
It becomes compulsory once your business turnover reaches $75,000 a year, with 21 days to register from that point, per the ATO. Below that threshold, registration is optional.
Sources
Electricity rates: AER Default Market Offer (NSW, SA, south-east QLD), Victorian Default Offer, and each remaining state's regulated tariff, current for 2026–27. Appliance figures: the energyrating.gov.au registration database (July 2026 snapshot). Methods and assumptions are described on each page.