One of the more popular FAQs, calculating sales tax without a calculator can be done using basic arithmetic. First, determine the sales tax rate. Then convert that to a decimal by dividing the tax rate by 100. For example, if you have a tax rate of 8%, the decimal would be .08. Next, take the pre-tax amount of the item and multiply it by the decimal. For example, if the item is $50, you have $50 x .08 = $4. So your sales tax amount is $4, with the total purchase amounting to $54. Keep in mind, this basic method only provides an estimate. The actual tax may vary slightly due to rounding or if the tax rate is not a whole number.