This page answers the four percentage questions people actually run into: what is X% of a number, what percent one number is of another, how much something went up or down in percent, and what you get after adding or taking off a percentage. Each answer shows the working underneath, so you can check it or copy the method.
It is handy for discounts in a sale, a salary raise, tax on a bill, exam marks, or working out how much a price really jumped since last year.
How to use
- Find the box that matches your question. Each one works on its own.
- What is X% of a number? Type the Percent (%) and the number in Of. The answer appears on the right.
- X is what percent of Y? Enter the part in X and the whole in Y.
- Percentage increase or decrease Enter the old value in From and the new value in To. The label changes to Increase or Decrease to match.
- Add or subtract a percentage Enter the Amount and the Percent (%). You get both results side by side.
All fields come prefilled with an example, so just type over them.
The formulas behind each box
Percent of a number is percent ÷ 100 × number. 15% of 2,400 is 0.15 × 2,400 = 360.
What percent X is of Y is X ÷ Y × 100. If you scored 360 out of 2,400, that is 15%.
Percentage change is (new − old) ÷ old × 100. A price that went from 1,200 to 1,500 rose by 300, and 300 ÷ 1,200 is 0.25, so that is a 25% increase. Always divide by the old value, not the new one.
Adding or removing a percentage multiplies by (1 + p ÷ 100) or (1 − p ÷ 100). Adding 18% to 5,000 gives 5,000 × 1.18 = 5,900. Taking 18% off gives 5,000 × 0.82 = 4,100.
Mistakes that trip people up
The most common one: a 20% rise followed by a 20% drop does not bring you back to where you started. Start at 100, add 20% to get 120, then take 20% off 120 and you land on 96. The second percentage is applied to a bigger number.
Another is removing tax the wrong way. If a price of 1,180 already includes 18% tax, the price before tax is not 1,180 minus 18%. It is 1,180 ÷ 1.18 = 1,000. Taking 18% off 1,180 gives 967.60, which is wrong.
People also mix up percent and percentage points. If an interest rate goes from 10% to 12%, it rose by 2 percentage points, but by 20% in relative terms. Banks and news reports often use whichever sounds better.
FAQ
How do I calculate a percentage of a number?
Divide the percent by 100 and multiply by the number. For 15% of 2,400, that is 0.15 × 2,400 = 360. The first box does this for you.
How do I work out percentage increase?
Subtract the old value from the new one, divide by the old value, and multiply by 100. Use the Percentage increase or decrease box and it shows the working.
How do I find a price before a discount?
If you paid 800 after a 20% discount, divide by 0.8 to get 1,000. Dividing works here because the discount was taken from the original price, not the final one.
Is this calculator free?
Yes. It runs in your browser, works offline once loaded, and nothing you type is sent anywhere.