This handles the two percentage questions people run into most, finding what a percentage of a number is, and finding what percentage one number is of another. Both are simple in theory, but easy to fumble under time pressure, whether you're splitting a discount or checking a grade.
How to use it
- For what is X percent of Y, enter the percentage in the first field and the base number in the second.
- Read the result right below.
- For X is what percent of Y, enter the value and the total in the second section.
- Read the percentage below that.
- Change any value anytime and both results update instantly.
When it helps
- Figuring out a discounted price during a sale.
- Checking what percentage of a test you scored.
- Working out what share of a monthly budget one expense represents.
- Calculating a percentage increase or decrease between two figures, like sales year over year.
- Working out a tip as a percentage of a restaurant bill before splitting it.
Good to know
- A percentage increase and the matching decrease aren't symmetrical. A 50 percent increase followed by a 50 percent decrease doesn't bring you back to where you started.
- Double check which number is the whole and which is the part before calculating. Swapping them gives a very different, wrong answer.
- For anything involving compounding, one calculation isn't enough. You'd need to apply the percentage repeatedly across each period.
- When comparing several percentage off deals, work out the actual final price for each rather than just comparing the percentages, since the starting prices are often different too.
- When working out a tip or a discount by hand as a sanity check, round the percentage to a friendlier number first, like treating 18 percent as roughly a fifth, to catch an obviously wrong calculator result.
Questions
How do I calculate a percentage increase between two numbers?
Subtract the original from the new number, divide by the original, then multiply by 100. The second section here can handle the division and multiplication once you've found the difference.
Why is 20 percent of 150 the same as 150 percent of 20?
Mathematically they both come out to 30. The confusion is usually about which number is the base, not the math itself.
Can this handle percentages over 100?
Yes, there's no upper limit, which is useful for calculating a markup or growth that exceeds the original value.
How do I calculate a percentage decrease?
Use the second section the same way. Enter the smaller, later value and the original larger value, and the result shows what percentage the later value represents, from which you can work out the decrease.
Can I chain multiple percentage calculations together?
This handles one calculation at a time. For a multi step calculation, like a discount followed by tax, run each step through separately, using the previous result as the next input.
Can I calculate a percentage of a negative number?
Yes, the math works the same way and gives a negative result, which is useful for something like calculating a loss rather than a gain.