Four common percentage questions
"Percent" means "per hundred": 15% is 15 out of every 100, or 0.15 as a decimal. Almost every percentage question comes down to one of four calculations.
1. What is X% of a number?
result = number × X ÷ 100
Example: 15% of 240 = 240 × 15 ÷ 100 = 36. This is how you work out a tip, a commission or a tax amount.
2. X is what percent of Y?
percentage = X ÷ Y × 100
Example: you scored 45 out of 180 points. 45 ÷ 180 × 100 = 25%. Use this for test scores, budget shares and conversion rates.
3. Add a percentage to a number
result = number × (1 + X ÷ 100)
Example: a $50 price plus 20% is 50 × 1.20 = $60. Use it for price increases, markups and sales tax.
4. Subtract a percentage from a number
result = number × (1 − X ÷ 100)
Example: $50 minus 20% is 50 × 0.80 = $40. Discounts work the same way; the discount calculator also handles stacked discounts.
Mental math shortcuts
- 10%: move the decimal point one place to the left. 10% of 360 is 36.
- 5%: half of 10%. 5% of 360 is 18.
- 1%: move the decimal point two places. 1% of 360 is 3.6.
- 15% or 20% tip: take 10% and add half of it (15%) or double it (20%). On a $64 bill: 10% = $6.40, so 15% ≈ $9.60 and 20% = $12.80.
- Swap trick: X% of Y equals Y% of X. 8% of 50 is the same as 50% of 8, which is 4.
Common mistakes
- Adding and then removing the same percentage does not return the original number. $100 + 20% = $120, but $120 − 20% = $96. To undo a 20% increase, divide by 1.20.
- Percent versus percentage points. If an interest rate goes from 4% to 5%, it rose by 1 percentage point, but by 25% in relative terms. Use the percentage change calculator for relative changes.
- Percentages of different bases can’t simply be added. A 10% discount on one item and 20% on another is not a 30% discount overall.
Where you use percentages
Percentages appear everywhere: sales tax and VAT (see the VAT calculator), interest rates, discounts, grades, nutrition labels, survey results and profit margins. Being able to switch between fractions, decimals and percentages makes comparing offers much easier: 3/8 = 0.375 = 37.5%.
Percentages, fractions and decimals
Percentages, fractions and decimals are three ways to write the same thing. To turn a percentage into a decimal, divide by 100: 7.5% = 0.075. To turn a decimal into a percentage, multiply by 100: 0.42 = 42%. A fraction becomes a percentage when you divide the numerator by the denominator and multiply by 100: 3/8 = 0.375 = 37.5%. Some useful ones to remember: 1/2 = 50%, 1/3 ≈ 33.3%, 1/4 = 25%, 1/5 = 20%, 1/8 = 12.5% and 1/10 = 10%. Knowing these by heart makes it much easier to check whether a calculated percentage is plausible, for example when a store claims "a third off" or a report says "one in eight".
Percentages above 100%
A percentage can be larger than 100. If a price goes from $40 to $100, the new price is 250% of the old price, and the increase is 150%. Mixing up "of" and "increase" is a common source of confusion; for increases, use the percentage change calculator.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100. For example, 30% of 80 is 80 × 30 ÷ 100 = 24.
How do I find what percentage one number is of another?
Divide the part by the whole and multiply by 100. 18 out of 24 is 18 ÷ 24 × 100 = 75%.
How do I add 20% to a price?
Multiply by 1.2. A $45 item plus 20% costs $54.
How do I remove 20% that was already added?
Divide by 1.2, don’t subtract 20%. If $60 includes a 20% markup, the original price was $60 ÷ 1.2 = $50.
Can I use negative numbers?
Yes. The calculator accepts negative values, for example to work out a percentage of a loss.
What is 1% of a million?
1% of 1,000,000 is 10,000.
Last reviewed: 2026-10-06. Results are estimates for information only.