How do you calculate the sales tax percentage?

Calculate sales tax percentage by multiplying the percentage of the sales tax by the total cost of the item(s) subject to the tax, states Basic-mathematics.com. To find the tax percentage rate on a purchase using only the subtotal and amount of tax, divide the total cost by the pre-taxed amount.

Figuring the total amount of sales tax requires multiplying the decimal representation of the tax by the item cost, according to Basic-mathematics.com. Convert a 5 percent sales tax to 0.05, and then multiply it by the amount of the sale. A $100 purchase times 0.05 produces a $5 sales tax.