Percentage Calculator: Percent of, Percent Change and More

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  • Runs in your browser
  • Last updated 2026-10-07
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Five common percentage questions in one place. Fill in two numbers and the answer appears at once.

What is X% of Y?

X is what percent of Y?

Percentage change from one number to another

Increase or decrease a number by a percentage

Find the original number

Everything you enter is processed in your browser and is never sent to a server.

How to use

  1. Find the box that matches your question: X% of Y, X is what percent of Y, percentage change, increase or decrease, or finding the original number.
  2. Type the two numbers. The answer appears as soon as both are filled in.
  3. Use a minus sign for a decrease in the last two boxes, for example -15 for a 15% decrease.

Examples

20% of 150
→ 30
30 is what percent of 150
→ 20%
From 80 to 100
→ an increase of 25%
From 100 to 80 is a decrease of 20%, not 25%, because the change is measured against the starting number.
Increase 200 by 15%
→ 230
To undo it, enter 230 and 15 under Find the original number to get 200.

How it works

A percent is a hundredth, so X% of Y is X divided by 100, times Y. What percent X is of Y is X divided by Y, times 100. Increasing a number by P% multiplies it by 1 plus P divided by 100, and undoing the change divides by the same factor.

Results are rounded to twelve significant digits, so that something like 0.1 + 0.2 shows as 0.3 and not 0.30000000000000004. They are shown with up to ten decimal places.

The tool is tested with the examples on this page, including the round trip that shows why a rise and a fall of the same percentage do not cancel out.

Sources

Frequently asked questions

How is a percentage change calculated?
It is the new value minus the old value, divided by the old value, times 100. Going from 80 to 100 is (100 - 80) / 80 = 25%. The starting number cannot be zero, because the change cannot be measured against zero.
Why does going up 20% and then down 20% not return to the start?
The second percentage is taken of a different number. 100 up 20% is 120, and 120 down 20% is 96. To return to 100 after a 20% rise you need a fall of about 16.7%, and a 25% rise needs a 20% fall.
What is the difference between percent and percentage points?
If a rate goes from 10% to 12%, it rose by 2 percentage points, but it rose by 20 percent in relative terms. The change between two percentages is usually best given in percentage points.
How do I find the original price before a discount?
Use Find the original number with the discount as a negative percent. A price of 170 after a 15% discount is entered as 170 and -15, and the original is 200.
Are my numbers sent anywhere?
No. All calculations run in your browser.

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