Percentage Calculator
Three percentage calculations in one place — a percent of a value, one value as a percent of another, and the percentage change between two values. The formula used is shown with every answer.
How to Use
- Pick the mode that matches your question: "What is X% of Y", "X is what % of Y", or "% change from X to Y".
- Type your two numbers into the input fields — the labels change to match the mode you selected.
- The result updates as you type, along with the exact expression that produced it.
- Use the copy button to put the answer on your clipboard, or Clear to start over.
Features
- Three separate percentage modes with a single set of inputs
- The evaluated formula is displayed beneath every result
- Handles negative numbers and decimals
- Guards against undefined cases such as dividing by zero or measuring change from zero
- Runs entirely in your browser — nothing is uploaded or stored
Understanding the Three Percentage Questions
Almost every percentage problem people meet in daily life is one of three questions wearing different clothes. The first is proportional: what is 15% of 200? You convert the percentage to a decimal by dividing by 100 and multiply, giving 0.15 × 200 = 30. This is the shape of a sales tax line, a commission on a sale, or a discount taken off a sticker price. The second question runs the other way: 30 is what percent of 200? Here you divide the part by the whole and multiply by 100, which returns 15%. Test scores, market share, and completion rates all live in this second category.
The third question — percentage change — is the one that trips people up most often, because it is not symmetric. Going from 200 to 250 is a 25% increase, but going back from 250 to 200 is a 20% decrease, not 25%. The reason is that the denominator changes: increase is measured against the original value, while the decrease is measured against the larger new value. This asymmetry is why a stock that falls 50% must then rise 100% just to break even, and why a "50% off, then 20% extra off" sale never adds up to 70% off. Our calculator always divides by the original value, so the direction of the comparison is unambiguous.
A related distinction worth knowing is percentage points versus percent. If an interest rate moves from 4% to 5%, that is a rise of one percentage point but a 25% relative increase. News reports blur these constantly, and the difference matters a great deal when the base number is small. When you use the change mode of this calculator, you are always getting the relative figure — the percent, not the percentage points — so if you want the point difference, just subtract the two rates directly.
Frequently Asked Questions
How do I calculate a discount price?
Use the first mode to find the discount amount — for a 30% discount on 89.99, enter 30 and 89.99 to get 27.00 — then subtract that from the original price. Alternatively, enter 70 instead of 30 and the result is the price you will actually pay.
Why does a 50% rise followed by a 50% fall not return me to the start?
Because each step is measured against a different base. 100 raised by 50% is 150; 150 reduced by 50% is 75, not 100. Successive percentage changes multiply rather than add, so 1.5 × 0.5 = 0.75.
Can I use negative numbers?
Yes. The change mode divides by the absolute value of the original number, so a move from −40 to −20 is reported as a 50% increase, which matches the everyday sense of a loss shrinking. Change measured from exactly zero is mathematically undefined, and the calculator says so instead of showing infinity.
How precise are the results?
Results are computed in standard double-precision floating point and displayed to at most six decimal places. For money you will normally want to round to two decimals yourself, since the calculator does not assume any particular currency.
Is my input sent anywhere?
No. The arithmetic runs in JavaScript inside your own browser tab. There is no server call, no logging, and nothing is retained after you close the page.