# Percentage Calculator - X% of Y, What Percent, Reverse Percentage, Add or Subtract

Page: https://hirantha.me/tools/percentage-calculator
Part of: Hirantha Sandaruwan Lakshan free tools (https://hirantha.me/tools)
Updated: 2026-10-13

Find X% of a number, what percent one number is of another, the whole from a part, or add and subtract a percentage, with the working shown.

This is a free tool that runs in the browser: open the page to use it. File tools process files on the visitor's own device; files are never uploaded or stored. No account, sign-up, payment or watermark.

Type two numbers and get the answer to the four percentage questions people ask most: what is X% of Y, X is what percent of Y, Y is X% of what number, and what a number becomes when you add or subtract a percentage. Each answer comes with the working in plain words and the formula, so you can check it or learn the method. Answers are exact, rounded only for display, and nothing you type leaves your browser.

A percentage is a fraction out of 100. To find X% of Y, divide X by 100 and multiply by Y: 15% of 80 is 0.15 x 80 = 12. The order does not matter, so 15% of 80 and 80% of 15 are both 12. That is handy when one way round is easier: 4% of 50 is the same as 50% of 4, which is 2.

To find what percent X is of Y, divide X by Y and multiply by 100: 36 / 240 = 0.15, so 36 is 15% of 240. The number after the word of is the whole, so it goes second. The answer can be more than 100% when X is bigger than Y (150 is 187.5% of 80). Y cannot be 0, because dividing by 0 has no answer.

To find the whole when you know a part and its percentage, divide the part by the percentage written as a decimal. If 45 is 15% of a number, the number is 45 / 0.15 = 300. Do not work out 15% of 45 and add it on: the 15% belongs to the unknown whole, not to the part you already know.

To add a percentage, multiply by 1 plus the percentage as a decimal; to subtract one, multiply by 1 minus it. Adding 12% to Rs. 2,500 is 2,500 x 1.12 = Rs. 2,800, and taking 18% off is 2,500 x 0.82 = Rs. 2,050. Adding a percentage and then subtracting the same percentage does not bring you back to the start, because the second percentage is of a different number: Rs. 2,500 plus 12% is Rs. 2,800, and 12% off Rs. 2,800 is Rs. 2,464.

The calculator reads each number exactly, as a fraction, and does every sum on fractions, so the rounding errors of ordinary computer arithmetic (0.1 + 0.2 giving 0.30000000000000004) cannot happen. Only the answer you see is rounded, to at most 6 decimal places with trailing zeros removed, and the page tells you when it has rounded. You can type commas, a leading Rs or a trailing % and they are ignored.

Worked examples (every answer is exact):
| Question | Working | Answer |
| --- | --- | --- |
| 15% of 80 | 15 / 100 x 80 | 12 |
| 12% of Rs. 2,500 | 12 / 100 x 2,500 | Rs. 300 |
| 10% of Rs. 45,000 | 10 / 100 x 45,000 | Rs. 4,500 |
| 7.5% of Rs. 1,200 | 7.5 / 100 x 1,200 | Rs. 90 |
| 2.5% of Rs. 80,000 | 2.5 / 100 x 80,000 | Rs. 2,000 |
| 25% of 640 | 25 / 100 x 640 | 160 |
| 36 is what percent of 240? | 36 / 240 x 100 | 15% |
| Rs. 750 is what percent of Rs. 2,500? | 750 / 2,500 x 100 | 30% |
| 50 is what percent of 1,250? | 50 / 1,250 x 100 | 4% |
| 45 is 15% of what number? | 45 / 0.15 | 300 |
| 42 is 7% of what number? | 42 / 0.07 | 600 |
| Rs. 12,000 is 8% of what amount? | 12,000 / 0.08 | Rs. 150,000 |
| Add 12% to Rs. 2,500 | 2,500 x 1.12 | Rs. 2,800 |
| Subtract 18% from Rs. 2,500 | 2,500 x 0.82 | Rs. 2,050 |

Q: How do I calculate a percentage of a number?
A: Divide the percentage by 100 and multiply by the number. For 15% of 80, 15 / 100 = 0.15 and 0.15 x 80 = 12. For 12% of Rs. 2,500 it is 0.12 x 2,500 = Rs. 300.

Q: How do I find what percent one number is of another?
A: Divide the first number by the second and multiply by 100. 36 is 15% of 240 because 36 / 240 = 0.15. The number after the word of is the whole, so put it second. If you swap them you get a different answer: 240 is 666.666667% of 36.

Q: How do I reverse a percentage and find the original number?
A: If you know a part and the percentage it stands for, divide the part by the percentage as a decimal. If 45 is 15% of a number, the number is 45 / 0.15 = 300. If a total of Rs. 1,150 already includes a 15% increase, the amount before was 1,150 / 1.15 = Rs. 1,000. Taking 15% off 1,150 would give Rs. 977.50, which is wrong because the 15% was of the original.

Q: How do I add a percentage to a number?
A: Multiply by 1 plus the percentage as a decimal. To add 12% to Rs. 2,500, multiply by 1.12 to get Rs. 2,800. To subtract 18%, multiply by 0.82 to get Rs. 2,050. The calculator shows this shortcut in its working.

Q: How do I work out a percentage increase or decrease between two numbers?
A: That is a different question: divide the change by the old number. From 80 to 100 the change is 20, and 20 / 80 = 25%, so it is a 25% increase. The percentage change calculator on this site does this and also shows the reverse.

Q: Does the order matter in X% of Y?
A: No. 15% of 80 is 12 and 80% of 15 is also 12, so you can swap the two numbers when one way is easier. This only works for finding a percentage of a number, not for what percent X is of Y.

Q: Can a percentage be more than 100%?
A: Yes. 150 is 187.5% of 80, and adding 150% to a number makes it two and a half times as large. A percentage above 100 just means the part is bigger than the whole you are comparing it with.

Q: How many decimal places does the calculator show?
A: Up to 6, with trailing zeros removed, so 12.500000 shows as 12.5. Some answers never end: 1 is 33.333333% of 3 when rounded to 6 places. When that happens the page says the answer is rounded. Everything before the final display is exact.

Q: Is anything I type sent to a server?
A: No. The sums run in your browser and nothing is stored or sent anywhere.

Sources: Percentage - Wikipedia (finding a percent of a number, the whole from a part such as 42 kg being 7%, a change of x percent giving 100 + x percent of the original), checked 2026-10-13 (https://en.wikipedia.org/wiki/Percentage); Percentage - Maths is Fun (percent of a number and what percent, worked examples), checked 2026-10-13 (https://www.mathsisfun.com/percentage.html)

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