Percentage Calculator is a free online tool to calculate percentages. What is % of? %. is what percent of? %. What is the percentage increase/decrease from to. Percentage Calculator. Calculate a percent of a value before and after, or find the percentage change between two values, and see how to calculate each one. Percentage calculator online to find percentage of a number, calculate x as a percent of y, find a number given percent. How to work out percentage formulas.