Percentage Calculator

Three common percentage formulas

Percentages are a way to describe a part out of one hundred, which makes them useful for discounts, taxes, commissions, grades, growth rates and comparisons. This calculator covers the three formulas people usually need. To find X% of Y, convert the percent to a decimal and multiply: Y × X ÷ 100. To find what percentage X is of Y, divide the part by the whole and multiply by 100: X ÷ Y × 100. To measure a percentage increase or decrease, compare the new value with the old value: (new − old) ÷ old × 100.

Because the result updates live, you can use the widget like a scratchpad. Try a sale price, change the tax rate, or compare two months of data without pressing a submit button. The formula line under the answer shows exactly which operation was used, so the number is easier to verify before you rely on it.

When to use each mode

Use the first mode for direct amounts: 18% tip on a bill, 7.5% sales tax, or 35% off a price. Use the second mode when you know the part and whole: 42 correct answers out of 50, 12 units sold out of 80, or a budget category as a share of total spending. Use the change mode when you have an old value and a new value, such as revenue moving from 1,200 to 1,500 or a price dropping from 90 to 72. Positive results mean an increase; negative results mean a decrease.

Privacy and accuracy notes

The calculator is fully client-side. Your entries are not uploaded, saved to an account or checked against an external service. JavaScript handles the arithmetic in your browser using standard decimal numbers. That is accurate for everyday percentage work, but you should still round according to the rules of your invoice, bank, school or report. If a starting value is zero in percent-change mode, the calculator asks for a non-zero value because division by zero has no meaningful percentage interpretation.