Percentages & Everyday Math
Percentage Calculator
Find what X% of a number is, or what percent one value is of another.
7 modes: percent of a number, percent from two values, find the whole, increase/decrease, percent change, reverse, and fractions.
Good for: quick math, price checks, ratios, and classroom work.
Output: answer with a chart, formula, and comparison values.
Result
Enter your numbers and press Calculate to see the result.
Calculating…
How to calculate percentages
A percentage expresses a number as a fraction of 100, which makes it easy to compare shares, discounts, and changes. This percentage calculator covers seven everyday cases: finding X% of a number, finding what percent one value is of another, finding the whole from a known part, increasing or decreasing a value by a percentage, measuring the change between two values, working backwards to an original value (reverse percentage), and converting fractions to percentages. Every answer comes with a chart — a donut for shares of a whole, and a before/after comparison for changes.
Formulas
X% of Y = (X ÷ 100) × Y
Part as a percent = (part ÷ whole) × 100
Whole from a part = part ÷ (percent ÷ 100)
Increase / decrease = base × (1 ± percent ÷ 100)
Percent change = ((end − start) ÷ |start|) × 100
Original value = final ÷ (1 + rate ÷ 100)
Fraction to percent = (numerator ÷ denominator) × 100
Worked examples
- 15% of 200 — (15 ÷ 100) × 200 = 30. The remaining value is 170.
- 30 as a percent of 200 — (30 ÷ 200) × 100 = 15%.
- 30 is 15% of what? — 30 ÷ 0.15 = 200.
- Increase 100 by 20% — 100 × 1.20 = 120; decrease instead: 100 × 0.80 = 80.
- Change from 100 to 125 — (25 ÷ 100) × 100 = +25%.
- Reverse: 125 after a 25% increase — 125 ÷ 1.25 = 100 was the original value.
- 3/8 as a percentage — (3 ÷ 8) × 100 = 37.5% (decimal 0.375).
Everyday situations where percentages come up
- Shopping — a "25% off" tag, a "buy one get 50% off the second" deal, or comparing unit prices.
- Tips and tax — adding an 18% tip or working out how much of a receipt is sales tax.
- Grades and tests — turning 47 correct out of 60 into a score (78.3%).
- Work and finance — a 4% raise, a commission rate, or interest on savings.
- Statistics — expressing "312 of 1,450 respondents" as 21.5% to compare groups fairly.
Quick mental-math shortcuts
You rarely need a formula for round numbers. To find 10%, move the decimal one place left (10% of 240 is 24). 1% moves it two places (1% of 240 is 2.4). Build the rest from those blocks: 20% is double 10%, 5% is half of 10%, and 15% is 10% plus 5%. Percentages are also reversible — 8% of 50 equals 50% of 8 (both are 4), so flip the problem whenever one side is easier.
Percentage points vs. percent change
These are easy to confuse. If an interest rate rises from 5% to 7%, that is a jump of 2 percentage points, but a 40% increase in the rate itself (2 ÷ 5). News headlines often mix the two, so check whether a figure describes the gap between two percentages or the change relative to the starting value.
For tracking growth or decline over several periods, try the percentage increase/decrease calculator, and for sale prices see the discount calculator.
Percentage Calculator — frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number: X% of Y = (X ÷ 100) × Y. For example, 15% of 200 is (15 ÷ 100) × 200 = 30.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100: (part ÷ whole) × 100. For example, 30 is (30 ÷ 200) × 100 = 15% of 200.
What is a reverse percentage?
A reverse percentage finds the original value before a percentage change was applied: original = final ÷ (1 + rate ÷ 100). If a price is 125 after a 25% increase, the original was 125 ÷ 1.25 = 100.
What is the difference between percentage and percentage points?
Percentage expresses relative change, while percentage points express the absolute difference between two percentages. Going from 10% to 15% is a rise of 5 percentage points but a 50% relative increase.
How do I find the whole if I know the part and the percent?
Divide the part by the percent expressed as a decimal: whole = part ÷ (percent ÷ 100). If 30 is 15% of some number, that number is 30 ÷ 0.15 = 200.
How do I increase or decrease a number by a percentage?
Multiply by (1 + percent ÷ 100) to increase or (1 − percent ÷ 100) to decrease. Increasing 100 by 20% gives 100 × 1.20 = 120; decreasing gives 100 × 0.80 = 80.