Figure your total sales, writing down the total price of the item or items. For example, you may have sold an item for $100.
Take your state or municipality's current sales tax rate and write it down. For example, a typical rate could be 5.75%.
Add the current sales tax rate to 100 to come up with a new percentage. For example, if the rate is 5.75 percent, the resulting figure would be 105.75 percent. Divide the percentage by 100 before making any further calculations. In this example, the result is 1.0575.
Divide your sales figure by the figure calculated in step 3. For example, $100 divided by 1.0575 percent equals $94.56. This is the price of the item without sales tax.
Subtract the price you calculated in step 4 from the original price. In this continuing example, $100 minus $94.56 equals a sales tax total of $5.44.