What GST is and how it is calculated
GST stands for Goods and Services Tax, a consumption tax that is charged as a percentage of the price of goods and services. Different countries use different rates, and some apply several rates to different categories of goods. This tool does not know any country's rules. It simply applies whatever percentage you type.
There are two directions to the math. To add GST, multiply the base price by the rate divided by 100 and add that to the base. To remove GST from a price that already includes it, divide the total by 1 plus the rate as a decimal. The second case is the one people get wrong: you cannot just subtract the percentage from the total, because the tax was calculated on the smaller base amount.
When you need to add or remove GST
Sellers need to add GST when they quote a price to a customer, and buyers often need to remove it to find the pre-tax cost of something they were quoted in full. Bookkeeping is the other common case: when you have a receipt showing only the total, you may need to split it into the base and the tax portion for records or a return.
Freelancers and small shops also use this to check that an invoice is internally consistent, for example confirming that the tax line equals the stated rate applied to the subtotal.
How to use the GST calculator
- Choose Add GST if your amount does not include tax, or Remove GST if it already does.
- Enter the amount. The label changes to "Amount (excl. GST)" or "Amount (incl. GST)" so you know which one you are supplying. It starts at 1000.
- Enter the GST rate (%). It starts at 18.
- Read the three results: Base amount, GST amount, and Total.
Worked examples: adding 18% to 1000 gives a base of 1000.00, GST of 180.00 and a total of 1180.00. Removing 18% from 1000 gives a base of 847.46, GST of 152.54 and a total of 1000.00. Check: 847.46 x 1.18 is about 1000.
What the tool does not do
It works with a single rate at a time. It does not split GST into central and state parts, handle several items at different rates, or round each line the way an invoicing system might, so a total built from many items can differ by a few cents from a figure produced line by line.
Results show two decimal places with no currency symbol, which works for any currency. Always confirm the correct rate for your goods and region from an official source, because this tool takes the rate on trust. For general percentage questions you can use the Percentage Calculator.