With a regular calculator using basic math functions, it is possible to calculate percentages. To calculate percentages on a regular calculator, find the difference between the two numbers be compared, then divide that number by the original number and multiply by 100. The result is a percent increase or decrease.
Some basic calculators have a percentage button included. There are two ways for these buttons to calculate values depending on how the calculator was programmed. The percentage button either reads the input as the percent in question i.e. 90 [+] 10 [%] = 99, or it can interpret that input to the value of the fraction, as in this example 90 [+] 10 [%] = 90.1.