The two formulas — and the classic mistake
Adding GST is easy: multiply the GST-exclusive price by 1.1. Extracting it is where people slip: the GST inside a GST-inclusive price is the total divided by 11, not 10% of the total. A $1,100 invoice contains $100 of GST — because the $1,000 base plus 10% equals $1,100. Dividing by 10 instead of 11 overstates GST by 10% and quietly corrupts BAS figures; it's the most common bookkeeping error in Australian small business.
When GST applies to you
Registration is required once your business turnover reaches $75,000 a year ($150,000 for non-profits), and it's optional below that. Once registered, you add GST to taxable sales, claim credits for GST paid on business purchases, and remit the difference on your BAS. The GST you collect was never your money — treating it as income is how small businesses end up with BAS bills they can't pay. GST-free categories include most basic food, health, education and exports; input-taxed categories (like residential rent and most financial services) carry no GST but also give no credits.