Percentage Calculator
Six percentage questions, one tool: what is X% of a number, what percent one number is of another, what whole a part belongs to, the percent change between two numbers, a value increased or decreased by a percent, and the symmetric percent difference. Every mode gives a plain-English answer plus the worked formula. All in your browser.
The six questions percentages answer
Almost every percentage problem is one of a few shapes. "What is X% of Y" finds a part from a percentage — a tip, a tax, a discount. "X is what % of Y" turns a part and a whole into a rate — a score, a completion, a share. "X is Y% of what" recovers the whole from a part — the full price behind a deposit. "Percent change" measures movement between two numbers — a price rise, a growth rate, a drop. "Increase / decrease by %" applies a change to get the new value — a raise, a markdown. And "% difference" compares two values symmetrically when neither is the starting point. Pick the shape that matches your question and the arithmetic — shown under every result — takes care of itself.
The percentage that doesn't come back
The most common percentage mistake is assuming increases and decreases are symmetric. They aren't, because each applies to a different base. A 50% loss followed by a 50% gain doesn't return you to where you started — it leaves you down 25%. That's why a stock that falls 50% needs to double (a 100% gain) just to break even. Percent change is always relative to the starting value.
Related
- Converters hub — units, timezones, currency, color, and more
- Tip calculator — percentages applied to a bill
- Discount calculator — percent off and final price
- Sales tax calculator — add or remove a tax percentage
FAQ
Is anything I enter sent to a server?
No. The calculator runs entirely in your browser — open DevTools → Network and confirm. Nothing you type is uploaded.
How do I find what X% of a number is?
Use the "% of a number" mode. To find 20% of 150, multiply: 150 × 20 ÷ 100 = 30. The shortcut is to move the decimal: 10% of any number is that number ÷ 10, and you can scale from there (20% is double 10%).
How do I find what percent one number is of another?
Use the "X is what %" mode: divide and multiply by 100. 30 ÷ 150 × 100 = 20%, so 30 is 20% of 150. This is how you turn a part-and-whole into a percentage — test scores, completion rates, market share.
How do I calculate percent change (increase or decrease)?
Use the "% change" mode: (new − old) ÷ old × 100. From 150 to 180 is (180 − 150) ÷ 150 × 100 = +20%. A positive result is an increase, negative is a decrease. Note it's always relative to the old value — that's why a 50% drop needs a 100% rise to get back.
Why doesn't a percent increase cancel the same percent decrease?
Because each percentage applies to a different base. Take 100, add 50% → 150. Now subtract 50% of 150 → 75, not 100. The increase was 50% of 100; the decrease was 50% of the larger 150. This asymmetry trips people up constantly — with investments, a 50% loss needs a 100% gain to recover. Try it yourself in the "Increase / decrease by %" mode.
How do I increase or decrease a number by a percentage?
Use the "Increase / decrease by %" mode: the new value is value × (1 ± percent/100). Increasing 150 by 20% is 150 × 1.20 = 180; decreasing it by 20% is 150 × 0.80 = 120. The calculator also shows the absolute change, so a 20% raise on 150 reads as +30 — useful for price rises, raises, and markdowns.
How do I find the whole when I know the part and the percent?
Use the "X is Y% of what" mode: divide the part by the percent as a decimal — whole = part ÷ (percent/100). If 30 is 20% of some number, that number is 30 ÷ 0.20 = 150. This is the reverse of "% of a number", and it's the right tool for questions like "the deposit was $500 and that's 10% — what's the full price?"
What's the difference between percent difference and percent change?
Percent change is directional: (new − old) ÷ old, measured against the starting value, so 100 → 150 is +50% but 150 → 100 is −33.3%. Percent difference is symmetric: |a − b| ÷ ((a + b)/2), measured against the average, so 100 and 150 differ by 40% in either order. Use change when one number comes before the other in time; use difference when you're just comparing two values side by side.