How it works
A percentage is a number out of 100, so 20% means 20 out of every 100. Almost every percentage question links three numbers: a whole, a part of it, and the percentage the part makes up. If you know any two, you can work out the third.
Pick the question you want to answer, enter the two numbers you know, and the calculator finds the third. The steps show exactly how, so you can do the same with a pencil next time.
The formula
All three questions come from one relationship:
part = percentage ÷ 100 × whole
- percentage is the number with the % sign (20 for 20%)
- whole is the total amount
- part is the piece of the whole the percentage describes
Rearranged, it answers each question:
| Question | Formula |
|---|---|
| What is 20% of 150? | part = 20 ÷ 100 × 150 |
| 30 is what % of 150? | percentage = 30 ÷ 150 × 100 |
| 30 is 20% of what? | whole = 30 ÷ (20 ÷ 100) |
Worked example
Want this with your own numbers? See Show the work above.
Here’s the same set of numbers, 20%, 30, and 150, worked out each way.
What is 20% of 150?
- 20% as a decimal is 20 ÷ 100 = 0.2.
- 0.2 × 150 = 30.
30 is what percent of 150?
- 30 ÷ 150 = 0.2.
- 0.2 × 100 = 20%.
30 is 20% of what?
- 20% as a decimal is 0.2.
- 30 ÷ 0.2 = 150.
FAQ
How do I work out a percentage in my head?
Start with 10%, which is the number divided by 10. For 20% of 150, 10% is 15, so 20% is twice that, 30. For 5%, halve the 10% figure. For 1%, divide by 100. Most everyday percentages are a mix of these.
Is 20% of 50 the same as 50% of 20?
Yes. Both are 20 ÷ 100 × 50 = 10, because the order of multiplication doesn’t matter. So if one way is hard, try the other: 8% of 25 is the same as 25% of 8, which is 2.
Can a percentage be more than 100%?
Yes. 100% is the whole amount, so more than 100% means more than the whole. 150% of 40 is 60, and 144 is 150% of 96.
How do I add or take off a percentage?
To add 20% to 150, multiply by 1.2 (that’s 100% plus 20%), which gives 180. To take 20% off, multiply by 0.8 (100% minus 20%), which gives 120. Taking 20% off and then adding 20% back doesn’t return you to the start, because the second 20% is worked out on a smaller number.
What’s the difference between percent and percentage points?
A percentage point is the plain difference between two percentages. If a rate goes from 4% to 5%, it rose by 1 percentage point, but by 25% of its old value, because 1 is 25% of 4.
How do I work out a percentage increase or decrease?
Divide the change by the starting value, then multiply by 100. Going from 40 to 50 is a change of 10, and 10 ÷ 40 × 100 = 25%, so it’s a 25% increase.
Sources
- Translating and solving percent questions: OpenStax Prealgebra 2e, section 6.2, Solve General Applications of Percent
- Finding a percent of a number, a percent from two numbers, and the whole from a part: OpenStax Contemporary Mathematics, section 6.1, Understanding Percent
Cite this page
APA (7th edition)
Mathlark. (2026, September 30). Percentage calculator. https://mathlark.com/math/percentage-calculator/
MLA (9th edition)
"Percentage Calculator." Mathlark, 30 Sept. 2026, mathlark.com/math/percentage-calculator/.