How Percentage Calculator Works
Each mode uses a different rearrangement of the same relationship between a part, a whole and a percentage. "X% of Y" multiplies Y by X/100. "X is what % of Y" divides X by Y and multiplies by 100. "Percentage change" subtracts the starting value from the ending value, divides by the starting value, and multiplies by 100 — a positive result is an increase, a negative result is a decrease.
How to Use This Tool
- Choose the calculation type that matches your question.
- Fill in the two number fields — their labels update to match the selected mode.
- Select Calculate to see the result.
- Switch modes at any time; your entered numbers are kept where they still make sense.
Example
X% of Y: 20% of 150 is 30. X is what % of Y: 30 is 20% of 150. Percentage change: going from 50 to 65 is a +30% change.
Helpful Tips
- A negative result in "percentage change" mode means a decrease — for example, 100 to 80 is -20%.
- To calculate a percentage increase or decrease directly, use the change mode rather than doing the subtraction by hand.
- For quick discount math, the discount calculator handles the price side of this automatically.
Frequently Asked Questions
What's the difference between "X% of Y" and "X is what % of Y"?
The first finds a value (for example, 20% of 150 is 30). The second finds a percentage (30 is what percent of 150? 20%). They're inverse operations.
Why does the change calculator show a negative number sometimes?
A negative result means the value decreased. For example, going from 200 to 150 is a -25% change.
Can I use decimal numbers?
Yes, all three fields accept decimals, such as 12.5.
Why do I get an error when calculating change from zero?
Percentage change is undefined when the starting value is zero, since you'd be dividing by zero. Use a non-zero starting value.
This tool runs entirely in your browser. Your input is not uploaded to any server.