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.