Enter a number, then click square root button. % Percent button is used to find the percentage of a number. Enter the percentage amount, click the % button, then enter the number you want the percentage of, and then click equals. i.e. 20% 125 = 25 where 25 is 20% of 125. Then use the upper calculator and type in -15 as the percentage change. On the other hand, if you happen to own 15% of the shoe shop, use the lower calculator and type in 15 as your percentage share. By using the alternative radio buttons to the right you can calculate different things depending on what information you already have. Percent. The percent function is used by clicking on the "%" or using the keyboard. The percent function is used to add, subtract, multiply, or divide a percent of a number. It's used to calculate the percent of a number. Here are some examples: 73+4.5% = 76.285. 18/80% = 1.25. Memory Functions