Percentage Calculator
Three calculators in one: find a percentage of a number, what percent one number is of another, or the percent change between two values.
How to use this calculator
Pick one of the three modes, enter the two numbers, and press Calculate. The calculator shows the formula with your numbers substituted, so you see exactly where the answer comes from, useful for tips, discounts, taxes, and grade math.
How it works
All three modes are one formula rearranged. “X% of Y” multiplies: (X ÷ 100) × Y. “X is what percent of Y” divides: (X ÷ Y) × 100. Percent change compares the difference to the starting value: ((Y − X) ÷ |X|) × 100, positive for increases and negative for decreases.
Worked example
Example: 20% off $150
Step 1: discount = (20 ÷ 100) × 150 = 0.20 × 150 = $30.
Step 2: sale price = 150 − 30 = $120.
As a percent change from 150 to 120: (120 − 150) ÷ 150 × 100 = −20%, a 20% decrease. The signs agree.
Common mistakes
- Percentage points vs percent: rising from 20% to 25% is a 5 percentage point rise but a 25% relative increase, different claims.
- Wrong base for change: percent change always divides by the starting value. Going 100 → 150 is +50%, but 150 → 100 is −33.3%, not −50%.
- Adding percentages: two 10% discounts are not 20% off, they compound to 19% off (0.9 × 0.9 = 0.81).
When to use it
Percentages price discounts, tips, taxes, interest, grades, poll swings, and business growth. Anytime a number is described “relative to a hundred,” this is the tool, and the percent-change mode settles every “how much did it grow?” question.
Frequently asked questions
Divide X by 100 and multiply by Y. 20% of 150 = 0.20 × 150 = 30.
Divide X by Y and multiply by 100. 30 is what percent of 150? (30 ÷ 150) × 100 = 20%.
Subtract the old value from the new, divide by the old value, multiply by 100. From 100 to 125: (125−100) ÷ 100 × 100 = 25% increase.
Yes, it just means the new value is more than double the old one. Going from 50 to 175 is a 250% increase.