Guide · The basics
How to Calculate a Percentage
Updated: June 2026
A percentage is just a number out of a hundred. Once you see it that way, every percentage question becomes the same small piece of arithmetic dressed up in different words. This guide walks through the one formula you actually need, the three shapes the question comes in, and a handful of mental-math shortcuts that make most everyday percentages doable without a calculator at all.
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What “percent” actually means
The word percent comes from the Latin per centum — “for each hundred”. So 25% means 25 out of every 100, which is the same as the fraction 25/100 and the decimal 0.25. Being comfortable moving between those three forms is the whole game. To turn a percentage into a decimal you divide by 100 (move the point two places left): 60% becomes 0.6, 7.5% becomes 0.075. To go back, multiply by 100.
The one formula
Almost everything reduces to a part, a whole, and a percent. Two of them are known, and you solve for the third:
percent = part ÷ whole × 100
whole = part ÷ percent × 100
Reading those three lines is enough to answer the three questions below. The calculator simply applies them for you and keeps the rounding tidy.
Question 1 — finding X% of a number
This is the most common one: what is 15% of 200? Convert the percent to a decimal and multiply. 0.15 × 200 = 30. If you prefer whole numbers, multiply first and divide last: 15 × 200 ÷ 100 = 30. Either order gives the same answer, so pick whichever keeps the numbers friendly in your head.
Question 2 — what percent one number is of another
Here you have the part and the whole and want the percent: 30 is what percent of 200? Divide the part by the whole and multiply by 100. 30 ÷ 200 = 0.15, and 0.15 × 100 = 15%. A useful sanity check: if the part is bigger than the whole, the answer must be over 100%.
Question 3 — percentage increase or decrease
When something changes from an old value to a new one, the percentage change measures the size of the move relative to where it started. Subtract the old from the new, divide by the old, and multiply by 100. Going from 80 to 100 is (100 − 80) ÷ 80 × 100 = 25% increase. From 100 down to 80 is (80 − 100) ÷ 100 × 100 = −20% — a decrease. Notice the two are not mirror images, because the starting point differs.
Mental-math shortcuts
- 10% — move the decimal one place left. 10% of 240 is 24.
- 5% — take 10% and halve it. 5% of 240 is 12.
- 20% — take 10% and double it. 20% of 240 is 48.
- 1% — move the decimal two places left, then scale up for 2%, 3% and so on.
- Swap trick — X% of Y equals Y% of X. 4% of 50 is awkward, but 50% of 4 is obviously 2.
A quick reference table
| Percent | Decimal | Of 200 |
|---|---|---|
| 1% | 0.01 | 2 |
| 5% | 0.05 | 10 |
| 10% | 0.10 | 20 |
| 25% | 0.25 | 50 |
| 50% | 0.50 | 100 |
| 75% | 0.75 | 150 |
For anything that doesn't fall on a round figure, the calculator handles the division and lets you set how many decimals you want to see.
Frequently asked questions
What is the formula for a percentage?
To find X% of a number, multiply the number by X and divide by 100. To find what percent one number is of another, divide the part by the whole and multiply by 100.
How do I work out 20% of a number in my head?
Find 10% by moving the decimal one place left, then double it. 10% of 70 is 7, so 20% is 14.
How do I turn a percentage into a decimal?
Divide by 100, which moves the decimal two places left. 45% becomes 0.45 and 7.5% becomes 0.075.
Can a percentage be more than 100?
Yes. Anything larger than the whole gives over 100%. If a price rises from 50 to 150, that is a 200% increase.