Percentage Calculator
Percentages come up everywhere — sale discounts, exam scores, tips, tax, raises, and statistics — yet the mental math is easy to fumble. This percentage calculator handles the three questions people ask most: what is X% of Y, X is what percent of Y, and what is the percentage change from one number to another. Pick the mode you need, type your numbers, and the answer appears instantly with a plain-English sentence.
Because each mode uses a different formula, mixing them up is the most common source of percentage mistakes. Keeping them separate here means you always apply the right one — whether you're checking a 25% discount, working out a test score, or measuring how much a price rose or fell.
How it works
Choose one of the three modes, then enter the two numbers it asks for. The calculator recomputes as you type — no button to press. Each mode answers a different question:
| Mode | Question it answers | Example |
|---|---|---|
| X% of Y | What is a percentage of a number? | 25% of 80 |
| X is what % of Y | What share of the whole is this part? | 20 out of 80 |
| Percentage change | How much did a value rise or fall? | From 50 to 75 |
Formula
Each mode uses one simple formula:
- X% of Y = (X ÷ 100) × Y
- X is what percent of Y = (X ÷ Y) × 100
- Percentage change from A to B = ((B − A) ÷ A) × 100 — positive means an increase, negative a decrease
Worked example
Suppose a $80 jacket is on sale at 25% off. First, find 25% of 80: (25 ÷ 100) × 80 = 20, so the discount is $20. Flipping it around, 20 is what percent of 80? (20 ÷ 80) × 100 = 25%. And if the price later climbs from $50 to $75, the percentage change is ((75 − 50) ÷ 50) × 100 = +50% — a 50% increase.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 to turn it into a decimal, then multiply by the number. For example, 25% of 80 is 0.25 times 80, which equals 20. A quick mental shortcut: find 10% by moving the decimal point one place left, then scale up or down — 10% of 80 is 8, so 25% is 2.5 times that, or 20.
How do I work out what percent one number is of another?
Divide the part by the whole and multiply by 100. If you scored 20 points out of a possible 80, that is 20 divided by 80, which is 0.25, so 25%. Make sure the whole goes in the denominator — swapping the two numbers is the most common mistake and gives a very different answer.
What is the formula for percentage increase or decrease?
Percentage change equals the new value minus the old value, divided by the old value, times 100. Going from 50 to 75 gives (75 minus 50) divided by 50 times 100, which is a 50% increase. Going from 80 to 60 gives negative 25%, meaning a 25% decrease. Always divide by the starting value, not the ending one.
What is the difference between percentage change and percentage points?
Percentage change is relative to the starting value, while percentage points measure the simple arithmetic difference between two percentages. If an interest rate rises from 4% to 5%, that is an increase of 1 percentage point, but a 25% percentage change, because 1 is a quarter of the original 4. News reports often mix these up, so check which one is meant.
Why is a 50% increase not undone by a 50% decrease?
Because each change is measured against a different base. Increasing 50 by 50% gives 75, but decreasing 75 by 50% gives 37.5, not 50. The decrease acts on the larger number, so it removes more in absolute terms. To exactly reverse a 50% increase you only need a 33.33% decrease, since 25 is one third of 75.