How to Calculate a Percentage
Learn the basic percentage formula with simple examples.
Published by QuickUtilaUpdated 3 min read
“Per cent” means “per hundred”. A percentage is simply a way of writing a number as a part of 100, so 25% means 25 out of every 100. Once you see percentages as fractions of 100, every percentage calculation comes down to one of three simple formulas.
1. Find a percentage of a number
This is the most common question: what is X% of Y? Divide the percentage by 100 to turn it into a decimal, then multiply by the number.
- 120 ÷ 100 = 0.20
- 20.20 × 500 = 100
Therefore: 20% of 500 is 100.
A useful shortcut: X% of Y is always the same as Y% of X. So 8% of 50 is the same as 50% of 8, which is 4.
2. Work out what percentage one number is of another
To find what percentage a part is of a whole — for example a test score of 42 out of 50 — divide the part by the whole and multiply by 100.
- 142 ÷ 50 = 0.84
- 20.84 × 100 = 84
Therefore: 42 out of 50 is 84%.
This is exactly what the Fraction to Percentage Calculator does: 42/50 is a fraction, and the calculator turns it into a percentage.
3. Find the original number from a percentage
Sometimes you know the percentage and the result but not the starting number. If 30 is 15% of a number, divide 30 by 0.15.
- 115 ÷ 100 = 0.15
- 230 ÷ 0.15 = 200
Therefore: 30 is 15% of 200.
Quick mental math tricks
- 10% — move the decimal point one place left: 10% of 340 is 34.
- 5% — half of 10%: 5% of 340 is 17.
- 1% — move the decimal point two places left: 1% of 340 is 3.4.
- 25% — divide by 4. 50% — divide by 2.
- Build others from these: 15% is 10% + 5%, and 35% is 25% + 10%.
Common mistakes to avoid
- Forgetting to divide by 100 — 20% is 0.20, not 20.
- Using the wrong “whole”. A 10% rise followed by a 10% fall doesn’t bring you back to the start, because the second 10% is taken from a larger number.
- Confusing percentage points with percent. A rate rising from 4% to 5% is a rise of 1 percentage point, but a 25% increase in the rate itself.