How it's worked out

GST in Australia is 10% of the price before GST.

Add GST: price × 10% = GST, and price + GST = total
Remove GST: total ÷ 11 = GST, and total − GST = price before GST

For example, $220 including GST has $20 of GST in it ($220 ÷ 11), so the price before GST is $200.

Common questions

Why divide by 11 instead of taking off 10%?: Because the 10% is worked out on the price before GST. In a $110 total, the GST is $10, which is one-eleventh of $110. Taking 10% off $110 would give $11, which is wrong.
Do I have to charge GST?: You must register for GST once your GST turnover reaches $75,000 a year ($150,000 for non-profits). Taxi and ride-share drivers must register whatever they earn. Below that, registering is your choice. See Registering for GST on the ATO website.
Does everything have 10% GST?: Most things do, but some are GST-free, such as most basic food, some health services and some education courses.