15% of 240 is 36. Pick the kind of question you have below, fill in X and Y, and the percentage calculator answers at once. It handles a share, a change or a price with a percent added.
Which question does each option answer?
Option one finds a part of a number, like a tip or a commission. Option two turns two numbers into a percent, which is what you want for a test score. Option three measures how far something rose or fell. Options four and five add a percent to a number or take one off, and that covers tax and discounts.
| Option | Example with X and Y | Result |
|---|---|---|
| X% of Y | 15% of 240 | 36 |
| X is what % of Y | 36 out of 240 | 15% |
| Percentage change from X to Y | from 80 to 100 | 25% |
| Percentage change from X to Y | from 100 to 80 | -20% |
| Add X% to Y | 250 plus 13% | 282.5 |
| Subtract X% from Y | 120 minus 30% | 84 |
How do you work out a tip, a raise or a sale price?
Say the restaurant bill is $240 and you want to leave 15%. Choose the first option, set X to 15 and Y to 240, and you’ll see 36. Flip the question with the second option, enter 36 and 240, and it hands back 15%.
Now rent goes from $80 to $100 a week. The third option shows a rise of 25%. If it drops back from $100 to $80, though, the change is minus 20%, not minus 25%, because the second sum starts from a bigger number. That one catches plenty of people out.
Raises work the same way. Go from $50,000 a year to $52,500 and the change option gives 5%, since the extra $2,500 is one twentieth of the old pay. To see what that does to a paycheque after tax, the salary increase calculator is the better tool.
To add 13% sales tax to $250, use the fourth option and you get 282.5, the same as the HST and GST calculator shows for Ontario. Taking 30% off $120 leaves 84, and the discount calculator will add tax to that sale price for you.
Why don’t 20% on and 20% off cancel out?
Because each percent is measured against a different starting number, and that’s the whole trick. Add 20% to 100 and you have 120. Take 20% off 120 and you’re at 96, not back at 100. Mixing up the base is the most common slip of all.
Percentage points are the other trap. A rate that moves from 4% to 5% has gone up by one percentage point, yet the change is 25%. The calculator doesn’t know what your numbers mean, since it treats X and Y as plain numbers, so it’s up to you to choose the option that fits your question, and that choice matters more than any digit you type. If X is zero in the change option, the answer shows 0, since a rise from nothing has no percentage. Results run to four decimal places at most, with trailing zeros dropped, so round to dollars and cents yourself if you need to.
How do I take the tax back off a total?
People often try the fifth option with 13% on a total of 282.5 and land on 245.78. That’s wrong. The 13% was added to the smaller pre-tax price, so you have to divide by 1.13 instead: 282.5 divided by 1.13 is 250. Same trap as the 20% above, just wearing a receipt. The HST reverse calculator does that division for you.
What is it good for day to day?
Checking a sale, scaling a recipe, seeing a raise as a share of pay, turning marks into a grade. Whatever the job, write down which number is the whole first, then pick the option that treats it as Y.
A tip on a $60 bill? Enter 18 and 60 in the first option and you get 10.8. To see how far a price has fallen, put the old price in as X and the new one as Y in the change option. A negative answer means it’s gone down.
Small prices work too. If your usual Timmies coffee creeps from $4.00 to $4.40, enter 4 and 4.4 in the change option and you’ll see a 10% jump. It feels like forty cents, but over a year of daily cups it’s a real chunk. Put the number on it before you grumble.
Sources, and how we checked them
Straight arithmetic on the two numbers you type. Nothing is looked up, and no rate or limit is built in. A percent of a number is X divided by 100, times Y. Change is the new value minus the old one, divided by the old one. We’re an independent site, and no government body is behind it.
The same maths sits behind other tools here. The salary increase calculator turns a raise into a percentage, and the profit margin calculator handles shares of a selling price.
About this calculator
- What it does
- Work out percentages, percentage change and add or subtract a percent.
- Category
- Math and Business Tools
- Last checked
- September 30, 2026
- Result type
- Estimate for information only, not tax advice.
Frequently asked questions
How do I find a percent of a number?
Choose X% of Y. For 15% of 240 you get 36.
How do I find what percent one number is of another?
Choose X is what % of Y. 36 out of 240 is 15%.
How is percentage change measured?
Take the new value minus the old one, then divide by the old one. From 80 to 100 it's 25%, and from 100 to 80 it's minus 20%.
How do I add tax to a price?
Choose Add X% to Y. Adding 13% to 250 gives 282.5.
Why are 20% on and 20% off not equal?
Each percent has a different base. 100 plus 20% is 120, and 20% off 120 is 96.
Sources and updates
Updated: . How we check rates · Editorial team · Updates