Percentage Calculator
What is X% of Y? Percentage change, increase, decrease — all in one tool.
This calculator handles the three percentage problems you'll ever meet: finding X% of a number (discounts, tips, tax), working out what percent one number is of another (test scores, proportions), and measuring the percentage change between two values (price rises, salary increases). Type your numbers and results appear instantly.
The formulas behind the tool
X% of Y = (X ÷ 100) × Y. X as a percent of Y = (X ÷ Y) × 100. Change from A to B = ((B − A) ÷ A) × 100. That last divisor — the original value — is where most manual mistakes happen, which is exactly why this tool exists.
Want to master the mental math?
Percentages are very learnable. Our guide Percentages Made Simple teaches the 10% shortcut, the "X% of Y = Y% of X" flip trick, and the reversal trap that catches almost everyone.
Worked examples of all three types
X% of Y — a 15% tip on a 68.40 bill. 15 ÷ 100 = 0.15, then 0.15 × 68.40 = 10.26. Total to pay: 78.66.
X as a percent of Y — 37 marks out of 45. 37 ÷ 45 = 0.8222, × 100 = 82.2%.
Change from A to B — rent rising from 850 to 910. (910 − 850) = 60, then 60 ÷ 850 = 0.0706, × 100 = 7.06% increase. Divide by the original 850, never the new 910.
The reversal trap
The most expensive percentage mistake, and it catches almost everyone: a 20% fall followed by a 20% rise does not return you to where you started.
100 falls by 20% to 80. That same 80 rising by 20% gives 96, not 100 — because the 20% rise is calculated on 80, not on the original 100. To undo a 20% fall you need a 25% rise.
The same applies to prices. If a jacket is 30% off at 70, the original was not 91 (70 + 30%). It was 70 ÷ 0.70 = 100. Work backwards by dividing, not by adding the percentage back.
Percent versus percentage points
These are different and the difference is not pedantry. If a rate moves from 4% to 6%, that is a rise of 2 percentage points — but a 50% increase. Both statements are true and they describe the same change. News reports and product marketing routinely use whichever sounds larger.
Two shortcuts worth memorising
- Find 10% first. Move the decimal one place left. 10% of 340 is 34, so 5% is 17, 20% is 68, and 15% is 34 + 17 = 51. Most everyday percentages can be built from 10% and 5%.
- X% of Y equals Y% of X. 4% of 75 is awkward; 75% of 4 is obviously 3. They are the same number. This one is genuinely useful and almost nobody is taught it.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. For example, 25% of 80 is (25 ÷ 100) × 80 = 20.
How do I work out percentage increase?
Subtract the old value from the new value, divide by the old value, and multiply by 100. Going from 50 to 65 is ((65 − 50) ÷ 50) × 100 = a 30% increase.
Why doesn’t a 20% increase then 20% decrease cancel out?
Because the decrease is applied to a larger number. 100 plus 20% is 120; 120 minus 20% is 96. Percentage changes are always relative to the current value, not the original.
Can I use this for discounts and tips?
Yes — use the "X% of Y" mode. For a 30% discount on $45, calculate 30% of 45 ($13.50) and subtract it, or calculate 70% of 45 directly to get the final price.