GST-inclusive and GST-exclusive prices
Goods and services tax (GST) is a value-added tax: it is charged as a percentage of the price without the tax. A price GST-exclusive leaves GST out and a price GST-inclusive has it included. Shops and suppliers quote either, so check which one you are looking at.
With an example rate of 10%, a GST-exclusive price of 50 has GST of 5 and a GST-inclusive price of 55. In the other direction, a GST-inclusive price of 110 is 110 ÷ 1.1 = 100 before GST, with 10 of GST. The calculator does either sum for the rate you enter.
The most common slip is to take the rate off the total. Ten percent off 110 gives 99, not 100, because the GST was 10% of the price before GST. Dividing by 1 plus the rate gives the right answer.
What a registered business can claim back
Like other value-added taxes, GST is meant to fall on the final customer. A business that is registered generally charges GST on its sales and claims back the GST it paid on business purchases, then pays the difference to the tax authority. The name of that claim, the registration threshold and the list of exempt goods and services differ by country.
This is why many suppliers quote GST-exclusive prices to businesses: the business can reclaim the GST, so the exclusive price is what the purchase costs it. A consumer pays the inclusive price.
Invoices with more than one part
In some countries the tax has more than one part, for example a national part and a local part, and an invoice may show them on separate lines or add them into one. If you know the combined rate, enter it. If the parts apply to different amounts, work out each part separately.
Rounding can differ by a cent between invoices, depending on whether the tax is worked out on each line or on the total. This calculator rounds once, on the amount you enter: the GST when you add it, and the price without GST when you take it out, with the GST as the difference. For tax in places that call it VAT, use the VAT calculator, which does the same sums with that word in the labels.