Percentage Calculator
Three modes: percent of a number, one number as a percent of another, and percent change. Live as you type.
What this tool does
Answers the three questions people actually mean when they say "percentage." Mode one finds a percentage of a number (what is 15% of 200). Mode two turns a part and a whole into a percentage (30 is what percent of 200). Mode three measures how much a value grew or shrank between two points (going from 200 to 250 is a 25% increase). Every field recomputes as you type, and empty or invalid entries are ignored rather than showing NaN. The math runs entirely in your browser; your numbers are never sent anywhere.
How to use it
Pick the section that matches your question and fill in its two boxes. The answer under that section updates on every keystroke, so there is no button to press. Decimals and negative numbers are fine. For example, typing 15 and 200 into the first section reads "15% of 200 = 30"; typing 30 and 200 into the second reads "30 is 15% of 200"; and typing 200 and 250 into the third reads "200 to 250 is a 25% increase." If you clear a box, that section simply waits until both numbers are back.
Common use cases
- Working out a tip, a discount, or a sales-tax amount from a bill total.
- Converting a test score or a subtotal into a percentage of the whole.
- Measuring growth or decline: revenue, weight, followers, prices, traffic.
- Checking a quoted "X% off" against the actual dollars you save.
- Sanity-checking a spreadsheet or a report before you send it.
Common pitfalls
- Percent change is not symmetric. A 50% increase followed by a 50% decrease does not return you to the start. Going 100 up to 150 and back down by 50% lands you at 75, because each step is measured against a different base.
- Percentages and percentage points differ. Moving from 10% to 15% is 5 percentage points but a 50% relative increase. Report whichever one your audience expects, and label it, so nobody reads a points change as a percent change.
- The base matters. "X% of Y" and "Y% of X" give the same number, but percent change is anchored to the starting value. Swap the from and to fields and the sign and size of the result both change. A change measured from a base of zero is undefined, which is why the tool declines to invent a number there.
Frequently asked questions
- How do I calculate what X percent of Y is?
- Multiply the whole by the percent divided by 100. So X% of Y equals Y times (X / 100). For example, 15% of 200 is 200 times 0.15, which is 30. The first mode of this tool does exactly that as you type, so you do not have to reach for a separate calculator.
- How do I find what percent one number is of another?
- Divide the part by the whole, then multiply by 100. So "X is what percent of Y" equals (X / Y) times 100. For example, 30 out of 200 is (30 / 200) times 100, which is 15 percent. The second mode handles this, which is handy for turning a raw score or a subtotal into a percentage.
- How do I calculate a percent increase or decrease?
- Subtract the old value from the new value, divide by the old value, then multiply by 100. So percent change from X to Y equals ((Y - X) / X) times 100. Going from 200 to 250 is ((250 - 200) / 200) times 100, which is a 25 percent increase. A negative result means a decrease. The third mode shows the direction in words.
- What is the difference between percent change and percentage points?
- They measure different things. If a rate rises from 10 percent to 15 percent, that is a rise of 5 percentage points, but it is a 50 percent relative increase (5 divided by 10). Use percentage points when you compare two percentages directly, and use percent change when you compare a new number against an old one. Mixing them up is one of the most common reporting errors.
- Why does the percent change say "undefined" when the starting value is zero?
- Percent change divides by the starting value, and dividing by zero has no meaning. There is no sensible way to express a change "relative to nothing" as a percentage, so the tool reports it as undefined rather than showing an infinite or misleading number. If your base is genuinely zero, describe the change in absolute terms instead.
- Does it handle decimals and negative numbers?
- Yes. You can type decimals like 12.5 or negative values like -40 in any field, and every mode updates live. Empty or non-numeric fields are ignored rather than crashing, so the result simply waits until both numbers are present. Nothing is rounded away until display, where results are shown to a sensible number of digits.
Cite this tool
For academic, journalistic, or technical references. Pick a format:
Citations use 2026 as the publication year. Access date is left as a fillable placeholder where the citation style expects one.