What is a percentage calculator?
A percentage calculator solves common problems that compare a part with a whole or measure how two values relate. The word percent means “per hundred,” so 25% is the same as 25/100 or 0.25.
This tool includes five calculation modes: percent of a number, one number as a percent of another, percentage change, percentage difference, and increasing or decreasing a value by a percentage. Each result includes the formula and the entered values substituted into it. For fractions, ratios, algebra and other related tools, visit the Math Calculators category.
How does the percentage calculator work?
Select a calculation type and enter the two requested values. The calculator validates the inputs, applies the matching percentage formula, and displays the result without changing the original entries.
The internal calculation uses JavaScript numbers and the full precision of your entries. The visible answer is rounded to a practical number of decimal places only at the final display step. Very large or very small valid results may use scientific notation to remain readable.
Some divisions are undefined. A ratio cannot use zero as the reference value; percentage change cannot start from zero; and percentage difference is undefined when both values have an average magnitude of zero. The calculator explains these cases instead of showing an invalid answer.
What percentage formulas does the calculator use?
What is X% of Y?
Example: 20% of 80 = (20 ÷ 100) × 80 = 16.
X is what percent of Y?
Example: 15 is 30% of 50 because (15 ÷ 50) × 100 = 30%.
Percentage change
A positive result indicates an increase; a negative result indicates a decrease. The absolute value in the denominator keeps the direction tied to the change in the numerator.
Percentage difference
Percentage difference treats both values symmetrically, so swapping A and B does not change the answer.
Increase or decrease by X%
Use plus for an increase and minus for a decrease.
How do you use this percentage calculator?
- Choose the calculation type. The field labels and example change to match your selection.
- Enter the first value. This may be a percentage, part, old value, first value, or base value.
- Enter the second value. This may be a number, whole, new value, second value, or percentage.
- Choose increase or decrease when using the adjustment mode.
- Select Calculate Percentage. Review the answer, explanation, formula, and inputs in the result panel.
You may enter whole numbers, decimals, zero, or negative values when the selected formula allows them. Do not type a percent sign inside a number field. Select Reset to return to the first mode and clear the calculator.
Percentage calculator examples
The same two numbers can produce different answers depending on the question. Choose the mode that matches the relationship you want to measure.
| Question | Inputs | Result |
|---|---|---|
| What is 15% of 240? | 15 and 240 | 36 |
| 45 is what percent of 60? | 45 and 60 | 75% |
| Change from 80 to 100 | 80 and 100 | 25% increase |
| Difference between 80 and 100 | 80 and 100 | 22.2222% |
| Decrease 250 by 12% | 250 and 12 | 220 |
Percentage change and percentage difference are not interchangeable. From 80 to 100, the change uses 80 as the starting reference. The difference uses the average of 80 and 100, which is 90, as a neutral reference.
Percentage change vs. percentage difference
Use percentage change when there is a clear original value and a later or revised value—for example, a price that rises from $50 to $60. The old value is the reference, so reversing the values changes both the magnitude and direction.
Use percentage difference when neither value is the original—for example, comparing two independent measurements. It divides the absolute gap by the average magnitude, so the answer is nonnegative and remains the same if the values are swapped.
Rounding, accuracy, and practical use
The calculator accepts decimal inputs and preserves their numerical precision during the operation. It removes unnecessary trailing zeros in the displayed result and limits long repeating decimals to keep the panel readable.
For schoolwork, show the formula and substitution before rounding. For prices, taxes, measurements, finance, science, or reporting, use the number of decimal places required by the specific context. A mathematically correct percentage can still be misleading if the reference value, time period, units, or sample is unclear.
This general-purpose tool does not apply specialized rules for sales tax, compound growth, margin, statistical uncertainty, or financial disclosures. Those calculations may require domain-specific inputs and rounding conventions.
Frequently asked questions about percentages
Definitions and methodology
This calculator implements the standard arithmetic definitions shown on this page. Percent means a ratio per one hundred. Percentage change uses the old value as its reference, while percentage difference uses the arithmetic mean of the two values’ absolute magnitudes.