How to Calculate Percentage
By Tools Star Hub Editorial Team · Published · Updated
To calculate a percentage, name the question first. What is X% of Y is Y times X divided by 100. 15% of 80 is 12, and the calculator’s detail line is 15 ÷ 100 × 80. X is what percent of Y is X divided by Y, times 100, and the second number cannot be 0. 12 is 30% of 40. A change applies the percent to a starting number: 200 increased by 10% is 220, and 80 decreased by 25% is 60.
Receipts, grades, and discounts use the same three shapes and different labels. Swapping the two numbers turns a discount into the wrong price. The Percentage Calculator keeps the shapes separate so you can match the sentence you actually need. A from-to change, such as 80 becoming 100, is the Percentage Change Calculator, which is a different page.
How do you find X percent of Y?
Divide the percent by 100, then multiply by the whole. 15 divided by 100 is 0.15, and 0.15 times 80 is 12. The page labels that result 15% of 80. Zero percent of 50 is 0. The percent can be larger than 100 if you mean more than the whole.
This mode does not ask which number is the part. You already named the percent. If you have a part and a whole and want the percent, use the next mode.
How do you find what percent one number is of another?
Divide the first number by the second and multiply by 100. 12 divided by 40 is 0.3, times 100 is 30. The page says 12 is this percent of 40 and shows the detail 12 ÷ 40 × 100.
The second number cannot be 0. Five of 0 is rejected with The second number cannot be 0 when finding a percentage. There is no percent of a zero whole. Zero percent of a nonzero number is a different mode, and that one is allowed.
How do you increase or decrease a number by a percent?
The change mode treats the first number as the percent and the second as the starting value. The amount is the percent of that start. An increase adds the amount. A decrease subtracts it. 200 increased by 10% has an amount of 20 and a result of 220. 80 decreased by 25% has an amount of 20 and a result of 60.
That is not the same as asking how much percent changed between two finished numbers. From 80 to 100, the Percentage Change Calculator reports a difference of 20 and a 25% increase. From 50 to 40, it reports a difference of -10 and a 20% decrease. Use that page when you already have the before and after.
How to calculate a percentage
- Write the question in one sentence. Use what is 15% of 80, 12 is what percent of 40, or what is 200 after a 10% increase. If you cannot tell which one it is, stop before you type.
- Pick the matching mode. Of, is-what, and change-by are separate. The change mode also needs increase or decrease.
- Check a round case. 10% of 50 is 5. If a quick case fails, the numbers are in the wrong boxes.
- Read the detail line. It shows the division the page used, such as 15 ÷ 100 × 80. The result is rounded to at most 10 decimal places, and the page displays up to six.
Example
15% of 80
The result is 12. The detail is 15 ÷ 100 × 80. That is the of mode: 15 is the percent and 80 is the whole.
A 25% decrease of 80
The amount removed is 20 and the result is 60. A $50 price falling to $40 is a different question: the change calculator calls that a difference of -10 and a 20% decrease.
Common mistakes
Zero cannot be the whole
You can take 0% of a number. You cannot ask what percent a number is of 0. The is-what mode rejects that.
A decrease by 20% is not a from-to change
Decreasing 50 by 20% gives 40. Seeing 50 and 40 and asking for the percent change is the other calculator, and the answer there is a 20% decrease as well, with a difference of -10. Keep the inputs in the mode that matches the sentence.
FAQ
- What is 15% of 80?
- 12. The calculator detail is 15 ÷ 100 × 80.
- 12 is what percent of 40?
- 30%. The detail is 12 ÷ 40 × 100. The second number cannot be 0 in this mode.
- What is 200 after a 10% increase?
- 220. The amount added is 20. A 25% decrease of 80 is 60, and the amount removed is 20.
- How do I get the percent from 80 to 100?
- Use the Percentage Change Calculator. From 80 to 100 the difference is 20 and the change is a 25% increase. From 50 to 40 the difference is -10 and the change is a 20% decrease.
- Do the numbers leave the browser?
- No. The Percentage Calculator runs in the browser.
To run these three percent questions, use the Percentage Calculator. It keeps of, is-what, and change-by as separate modes.
Tax is one applied percent. See How to add sales tax.
A shared bill uses the same kind of percent. See How to calculate a tip.