Adding tax and removing it are different sums
To add tax, multiply the net price by one plus the rate. To find the tax inside a price that already includes it, divide by one plus the rate, not multiply by the rate. Taking 20% off a VAT-inclusive price gives the wrong answer; dividing by 1.2 gives the right one. Both calculators work in either direction.
Sales tax, VAT and GST
In the United States, sales tax is set by states and often by counties and cities, so the rate depends on where the sale is delivered. Most other countries use a value-added tax, called VAT in the UK and Europe and GST in places such as Canada, Australia and New Zealand, charged at each stage of the supply chain at a national rate, sometimes with reduced rates for certain goods.
Rates and registration rules change, so confirm the rate that applies to your sale with the tax authority where you trade.