How to add and remove GST
GST (goods and services tax) and its cousins HST, PST, and QST are consumption taxes added to most sales. There are two everyday calculations: adding tax to a pre-tax price, and working backward from a tax-inclusive total to find the original price and the tax inside it.
add: total = price x (1 + rate)
remove: price = total / (1 + rate)The reverse calculation trips people up: you cannot just subtract the rate from a tax-inclusive total. To strip 10% out of a gross figure you divide by 1.10000, not multiply by 0.90.
Rates by region
Rates vary by country and, in Canada, by province. Australia charges a flat 10% GST and New Zealand 15%. In Canada, some provinces combine the 5% federal GST with a provincial tax (PST or QST), while others roll everything into a single HST. This calculator is set to Australia (GST 10%); switch the region above to match where the sale happens.
Who charges and claims it
Registered businesses add GST to sales, collect it from customers, and remit it to the tax authority, while claiming back the GST they paid on their own purchases. That is why invoices show the tax separately. If you are quoting a price, be clear whether it is tax-inclusive or tax-exclusive, since the difference is exactly what this tool calculates.