Percentage calculator

Choose the question before entering numbers

1. Select the mode whose sentence matches your problem.
2. Enter the values requested for that mode. Type 15 to mean 15%, without the percent sign.
3. Select Calculate. The result card shows the answer, direction where relevant, and the formula.
4. Use Copy result if you need the labeled answer elsewhere, or Clear to remove inputs and keep the selected mode.

Match common percentage questions to a mode

For “what is 15 percent of 80?” or “calculate 15% of 80,” use Percent of a number. For “30 is what percent of 120?” or “what percentage is 30 out of 120?”, use Part as a percent of whole. For “30 is 12 percent of what number?” or “find the original amount before a percentage,” use Find the whole.

For “percentage increase from 40 to 50,” “percentage decrease from 50 to 40,” or “how much did X change to Y?”, use Percentage change. For “percent difference between 40 and 60,” use Percentage difference when neither number is the original value. For “20% to 30% change,” choose Percentage-point change to see both the 10-point gap and the 50% relative increase.

Use Percent error for searches such as “observed versus actual percent error” or “measurement error percentage.” Use Progress percentage for “30 out of 50 completed,” “completion percentage,” “goal progress,” or “what percent is finished?” These phrases describe different formulas, so choose by the relationship between values rather than the word percent alone.

Parts, wholes, and reverse percentages

“Percent of a number” multiplies a value by a rate. “Part as a percent of whole” divides the part by its reference whole. “Find the whole” reverses that relationship when a part and its percentage are known. The labels matter: swapping the part and whole changes the answer.

These modes accept decimals and signed values. A negative result can be mathematically correct, such as −20% of 50 = −10, but a negative “whole” or “part” may be nonsensical for counts, physical quantities, or a particular report. The tool warns instead of guessing your domain.

Change is not the same as difference

Percentage change has a direction and a baseline: (new − start) ÷ |start| × 100. Moving from 40 to 50 is a 25% increase because the 10-unit rise is compared with 40. Moving from 50 to 40 is a 20% decrease because the baseline is now 50.

Percentage difference is symmetric. It compares the absolute gap with the average magnitude, so switching A and B does not change the result. For 40 and 60, the gap is 20 and the average magnitude is 50, producing a 40% difference. Use change for before-and-after data; use difference when neither value is the reference.

Percent error and progress

Percent error compares an observed value with an accepted or theoretical value. It reports the magnitude of the relative error and also shows the signed raw difference, observed minus accepted. An accepted value of zero cannot serve as the denominator. Percent error is not margin of error or standard error.

Progress divides completed amount by total. Values below 0% or above 100% are not automatically rejected because some schedules start with a deficit or continue beyond a target. A warning asks you to verify that interpretation. For ordinary task completion, use nonnegative values and a positive total.

Zero denominators, negatives, and rounding

• A whole, starting value, percentage used to find a whole, accepted value, or total cannot be zero when it is the divisor.
• Two zero values have no percentage difference because their average magnitude is zero.
• Negative values are calculated using absolute baseline magnitude for relative change and error, then flagged for interpretation.
• Results normally show at most two decimal places and remove trailing zeros. Very small nonzero results keep significant digits so they are not displayed as zero.
• Calculations retain full numeric precision internally; only the displayed and copied result is rounded.

Saving and limits

Copy result places the visible labeled answer on the clipboard. The calculator does not save a history, sync between devices, choose a currency, apply taxes or discounts, or make a decision from the result. If you need a record, paste the result and the original inputs into your own document.

JavaScript numbers are suitable for common everyday calculations, but not for arbitrary-precision scientific or accounting work. Extremely large or tiny inputs may lose precision. Confirm consequential results with the source data and the rules of your field.

Frequently asked questions