Adding sales tax to a total is very simple math. If your tax rate is 6% then that is the same as 0.06%. If you add 1 to the 0.06 you get 1.06% and times this number to your total purchase you will get the full cost of the item including sales tax. The 1 takes care of the price on the tag of the item and the 0.06 takes care of the tax. This formula is the fastest way to calculate. example: Item costs $10 x 1.06% tax = $10.60 total cost incuding tax.