How to use it
- Pick the calculator that matches your question, for example “X is what percent of Y?”.
- Type the numbers. Use a dot for decimals, such as 12.5.
- The answer appears immediately with the formula used, so you can copy it into a report or check it by hand.
How it works
A percentage is a number expressed as a fraction of 100, so 15% means 15/100 = 0.15. Every calculator here is a rearrangement of that one idea.
X% of Y = X × Y ÷ 100. X as a percentage of Y = X ÷ Y × 100. Percentage change from an old value to a new value = (new − old) ÷ |old| × 100; a positive result is an increase and a negative one a decrease. The absolute value of the old value is used so that a move from −10 to −5 counts as a 50% increase. A change from 0 is undefined, because any increase from nothing would be infinitely large.
Adding X% multiplies by (1 + X/100) and subtracting X% multiplies by (1 − X/100). Reverse percentage undoes that: if a price is 120 after a 20% increase, the original is 120 ÷ 1.2 = 100, not 120 − 20% = 96. This is the most common percentage mistake.
Results are rounded to 12 significant digits to hide tiny binary rounding errors (computers store 0.1 + 0.2 as 0.30000000000000004), then shown in your local number format.
Examples
Tip or commission
15% of 80 = 80 × 15 ÷ 100 = 12. A 15% tip on a bill of 80 is 12, so the total is 92 (or use “Add a percentage”: 80 × 1.15 = 92).
Price rise
A subscription went from 8.00 to 9.50. The change is (9.50 − 8.00) ÷ 8.00 × 100 = 18.75%, an increase of 18.75%.
Price before tax
A receipt shows 59.50 including 19% tax. The price before tax is 59.50 ÷ 1.19 = 50.00. Subtracting 19% from 59.50 would wrongly give 48.20.
Frequently asked questions
Why is a 20% increase followed by a 20% decrease not back to the start?
The second percentage is taken from a larger number. 100 + 20% = 120, and 120 − 20% = 96. To undo a 20% increase you divide by 1.2, which is a decrease of about 16.67%.
What is the difference between percentage change and percentage points?
If an interest rate rises from 4% to 5%, it went up by 1 percentage point, but the percentage change is (5 − 4) ÷ 4 × 100 = 25%. This calculator gives the percentage change; the difference line shows the change in points.
How do I calculate a percentage change when the old value is 0?
You can't: dividing by zero is undefined. Report the absolute difference instead (for example “from 0 to 40 sales”).
Can I use negative numbers or percentages over 100?
Yes. 150% of 40 is 60, and −20% of 50 is −10. For percentage change the old value's absolute size is used, so changes between negative numbers keep a sensible sign.
Privacy
All calculations run in your browser. The numbers you type are not sent anywhere or stored.