Percentage Calculator
Four percentage tools in one place: find what X% of a number is, work out what percent one number is of another, reverse-engineer a total from a percentage, and measure the percent increase or decrease between two values — each answering in a plain sentence as you type.
What is X% of Y?
Fill in both numbers to see the answer.
X is what % of Y?
Fill in both numbers to see the answer.
X is Y% of what?
Fill in both numbers to see the answer.
Percentage change
Fill in both numbers to see the answer.
How to Use Percentage Calculator
Pick the right card
Choose the card that matches your question — percent of a number, percent one number is of another, reverse percentage, or percent change.
Enter your two numbers
Fill in both inputs in the sentence.
Read the answer
The result appears instantly as a plain sentence, updating as you type.
About Percentage Calculator
The three classic percentage problems
Nearly every percentage question is one of three forms. 'What is 20% of 80?' multiplies: 80 × 0.20 = 16. '15 is what percent of 60?' divides: 15 ÷ 60 = 0.25 = 25%. '30 is 40% of what?' reverses it: 30 ÷ 0.40 = 75. The trick is recognizing which form you're facing — the unknown is either the part, the percent, or the whole. Each card on this page handles one form with its own inputs, so you never have to rearrange a formula in your head.
Percent change, done correctly
Percentage change compares a new value to an original: (new − old) ÷ old × 100. Going from 50 to 65 is a 30% increase; going from 65 to 50 is a 23.1% decrease — the asymmetry trips people up, and it's why a 50% loss needs a 100% gain to recover. The change is always measured against the starting value, which is also why percent change from zero is mathematically undefined (this calculator will tell you so rather than showing a misleading number).
Percentages vs. percentage points
One subtlety worth knowing: if an interest rate rises from 4% to 5%, that's an increase of one percentage point but a 25% relative increase — news reports regularly blur the two. Similarly, 'a 200% increase' means the value tripled (original plus twice the original), not doubled. This calculator always reports relative percentage change and states increase or decrease explicitly, so you can quote the result without ambiguity in reports, homework, or negotiations.
Common uses for Percentage Calculator
- Work out a sale discount or the original price before a markdown
- Convert a test score like 42 out of 50 into a percentage grade
- Measure the percent change in revenue, traffic, or weight between two periods
- Calculate a raise or price increase as a percentage for a report
- Reverse-engineer a total when you only know a partial amount and its percentage
Frequently Asked Questions
How do I calculate a percentage of a number?
Convert the percentage to a decimal by dividing by 100, then multiply. So 35% of 240 is 0.35 × 240 = 84. The first card does this automatically — enter 35 and 240 and read the sentence result.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100: if you scored 42 out of 50, that's 42 ÷ 50 × 100 = 84%. Order matters — the number after 'of' is always the denominator, which is why the second card labels its inputs as a sentence.
How does the reverse percentage calculation work?
If you know a part and the percentage it represents, divide the part by the decimal form: 18 is 30% of 18 ÷ 0.30 = 60. This is handy for reconstructing an original price from a sale amount — if you paid $45 and that was 75% of the original (25% off), the original was $60.
Why is percent change from 50 to 65 not the same as from 65 to 50?
Because change is measured relative to the starting value. From 50 to 65 the change is 15 ÷ 50 = 30% increase. From 65 to 50, the same 15-unit gap is divided by 65, giving a 23.1% decrease. This asymmetry is why a stock that drops 50% must gain 100% to break even.
Why can't I calculate a percentage change from zero?
Percent change divides by the original value, and division by zero is undefined. Going from 0 to any number isn't a percentage increase in the mathematical sense — it's often better described in absolute terms ('grew from 0 to 40 users') and the calculator flags this case rather than fabricating a number.