Percentage Calculator
To find X% of Y, divide X by 100 and multiply by Y: 15% of 200 = 0.15 x 200 = 30. To find what percent X is of Y, compute X / Y x 100; 30 out of 150 = 20%. Percentage change = (new - old) / |old| x 100, so 80 to 100 is a 25% increase while 100 to 80 is a 20% decrease.
This page covers the three percentage questions that come up outside the classroom: finding a percentage of a number, working out what share one number is of another, and measuring percentage change between two values. Pick a mode, enter your numbers, and the result appears with the arithmetic shown so you can check it by hand.
What is X% of Y?
X is what percent of Y?
Percentage change (increase / decrease)
How the three formulas work
Percentage of a number converts the percent into a decimal by dividing by 100, then multiplies. 15% of 200 is 0.15 x 200 = 30. Because "percent" literally means "per hundred", this conversion is the whole trick.
What percent X is of Y reverses the relationship: divide the part by the whole, then multiply by 100. With 30 out of 150, the calculation is 30 / 150 x 100 = 20%. The whole must be the denominator, and it cannot be zero.
Percentage change subtracts the old value from the new value, divides by the old value, and multiplies by 100. A positive answer is an increase; a negative answer is a decrease. This tool divides by the absolute old value so that a negative starting number still gives a sensible direction.
Worked example: a price and a discount
A jacket listed at 80 is marked up to 100. The change is (100 - 80) / 80 x 100 = 25%, so the price rose by a quarter. If a 20% discount is then applied to 100, the amount off is 0.20 x 100 = 20 and the new price is 80. Notice that a 25% rise followed by a 20% fall returns to the original value: the two percentages are not symmetric because each one is measured against a different base. A customer who paid 80 of a 100 budget has spent 80 / 100 x 100 = 80% of it, leaving 20%.
Edge cases and rounding
A zero whole is undefined for the "what percent" mode, and a zero old value is undefined for percentage change, so both are rejected instead of returning infinity. Negative inputs are accepted for the first two modes because debts and temperature changes are real; the change mode uses the magnitude of the old value as its denominator. Results are displayed to two decimal places without silently rounding the underlying value. Decimal inputs are allowed throughout. If you are comparing two percentages rather than two quantities, subtract them to get percentage points — dividing instead would give a percent change, which answers a different question.
Frequently asked questions
How do I find a percentage of a number?
Divide the percentage by 100, then multiply by the number. For 15% of 200, calculate 0.15 x 200 = 30. The same formula works for decimals and for percentages above 100, which simply produce a result larger than the original number.
Why is a 25% increase not reversed by a 25% decrease?
Because the base changes. Raising 80 by 25% gives 100, but lowering 100 by 25% gives 75, not 80. Percentage change always uses the starting value as its denominator, so equal-looking increases and decreases do not cancel out.
What is the difference between percentage points and percent change?
Percentage points describe a simple subtraction between two percentages: a rise from 10% to 12% is 2 percentage points. Percent change divides that difference by the original value, so the same move is a 20% increase. This tool computes percent change, not percentage points.
Are my numbers stored or sent anywhere?
No. All three modes run locally with JavaScript in your browser. Nothing you type is uploaded, saved to a database, or used for advertising, and reloading the page clears the inputs.
Last updated: 2026-11-10