Sales tax is a percentage of a purchase price charged by state and local governments in the United States and similar jurisdictions. It's added to the purchase price at the point of sale. Calculating sales tax accurately matters whether you're comparing prices, budgeting for a purchase, verifying a receipt, or running a business. The calculation is straightforward: multiply the pre-tax price by the sales tax rate. This guide covers how to add tax, remove tax from a total, and find effective combined rates.