Determine the gross amount you want to use before subtracting a percentage. On many bills or estimates, you may prefer the gross total before taxes have been applied. Subtracting 20 percent off a number that has additional taxes and fees might skew your overall estimate. Or if you are trying to subtract 20 percent from several different totals, you must either add up all the different numbers to get a final total or calculate each number individually.
Enter in the total amount by pressing the numbers on your calculator. As an example, assume the total is $85.50.
Press the "X" button, or the multiplication button, on the calculator.
Enter in "0.8" (80/100) to represent 80 percent, then hit the equal button. By multiplying a number by 80 percent, you automatically subtract 20 percent from the total. In the example, $85.50 x 0.8 = $68.40.