What this percentage calculator does
A percentage is a way to describe a part of a whole using 100 as the reference. This percentage calculator handles the four questions people ask most often: “What is a percentage of a number?”, “What percentage is one number of another?”, “How much did a value change?”, and “What happens when a percentage is added or removed?”
The tool is useful for school exercises, shopping comparisons, budgets, test scores, business reports, recipes, and quick checks in a spreadsheet. It keeps the inputs visible and prints the formula with the result, so you can see whether the base number belongs in the numerator or denominator. You can enter whole numbers, decimals, zero, and negative values when the calculation makes mathematical sense.
Use the result as an arithmetic reference. A percentage calculation does not decide how a store rounds a price, how a tax authority defines a taxable amount, or whether a financial rate is quoted as an annual or periodic rate. For those decisions, follow the rules attached to the original data.
Choose the percentage question that matches your task
| Question | Inputs | Formula | Example |
|---|---|---|---|
| What is X% of Y? | Percentage and base number | Y × X ÷ 100 | 15% of 240 = 36 |
| X is what percentage of Y? | Part and whole | X ÷ Y × 100 | 36 is 15% of 240 |
| How much did a value change? | Old value and new value | (New − Old) ÷ Old × 100 | 240 to 276 = 15% increase |
| Add or subtract X% | Percentage and starting value | Value × (1 ± X ÷ 100) | 240 − 15% = 204 |
If you are unsure which mode to use, first describe the sentence you want to complete. “15% of 240” uses the X% of Y mode. “36 is what percent of 240?” uses the X is what percentage of Y mode. “A price moved from 240 to 276” uses the percentage change mode.
How to use the percentage calculator
- Choose a question type. Select whether you are finding a part, a ratio, a change, an increase, or a decrease.
- Enter the first value. Depending on the mode, this is the percentage, the part, or the old value.
- Enter the second value. This is the base number, the whole, or the new value. The labels under the fields change when the mode changes.
- Select Calculate percentage. The result card updates with a readable answer, the selected question, and the formula used.
- Check the base. A percentage change needs a non-zero old value, and a “what percentage” calculation needs a non-zero whole. The tool will explain an invalid input instead of showing a stale answer.
Use Reset example whenever you want to return to 15% of 240. Because the calculation runs in your browser, you can change values repeatedly without waiting for a network request or creating an account.
Percentage formulas explained with examples
Finding a percentage of a number
To find 18% of 250, multiply 250 by 18 and divide by 100: 250 × 18 ÷ 100 = 45. The result, 45, is the part represented by 18% of the base number 250. You can also write 18% as 0.18 and calculate 250 × 0.18.
Finding what percentage one number is of another
To find what percentage 45 is of 250, divide the part by the whole and multiply by 100: 45 ÷ 250 × 100 = 18%. The order matters. The value being described goes on top, while the reference whole goes underneath.
Adding or subtracting a percentage
To add 15% to 240, first calculate the increase, 36, and then add it to the starting value: 240 + 36 = 276. To subtract 15% from 240, calculate the same 36 and subtract it: 240 − 36 = 204. The calculator shows the final value and the percentage amount used in the formula.
Percentage increase, decrease, and difference
Percentage change compares a new value with an old value. The old value is the reference, so the formula is (new value − old value) ÷ old value × 100. If the result is positive, the value increased. If the result is negative, the value decreased.
For example, moving from 80 to 100 gives (100 − 80) ÷ 80 × 100 = 25%, so the increase is 25%. Moving from 100 to 80 gives (80 − 100) ÷ 100 × 100 = −20%, so the decrease is 20%. The two directions are not symmetrical because each uses a different starting value.
Do not confuse percentage change with percentage points. If a pass rate moves from 40% to 55%, the change is 15 percentage points, but the relative percentage increase is 37.5% because 15 ÷ 40 × 100 = 37.5%. Use the wording that matches the question you need to report.
Input rules and edge cases
Zero as a base
Zero can be a valid base for “What is X% of Y?” because any percentage of zero is zero. Zero cannot be the whole in “X is what percentage of Y?” and cannot be the old value in percentage change, because division by zero has no defined percentage result.
Decimals and very small values
Inputs such as 12.5%, 0.75, or 0.004 are accepted. The answer is formatted to two decimal places for a stable display, while the calculation keeps the numeric value before formatting. For measurements or money, round again according to the convention used by your source.
Negative numbers and a decrease above 100%
Negative values can be meaningful in temperature, accounting, or signed-change data. Subtracting more than 100% from a positive value can produce a negative result; that is mathematically valid, but it may not make sense for a price, count, or physical quantity. Check the context rather than treating every result as a practical forecast.
Rounding and displayed precision
The page displays answers and percentage changes to two decimal places so that long decimals remain readable. A displayed 33.33% is a rounded presentation, not a statement that the underlying fraction is exactly 33.33%. Keep full precision in later calculations when the result will be reused.
Accuracy, privacy, and limitations
This is a deterministic calculator: it uses the formulas printed on the page and does not call an external calculation API. The browser performs the arithmetic locally, so the tool does not need your name, email address, or account. Do not enter confidential information on a shared computer if browser history or autofill could expose it.
Accuracy depends on choosing the correct reference value. A discount percentage normally uses the original price as its base. A growth rate normally uses the earlier value. A test score percentage normally uses the possible total. If you choose the wrong base, the arithmetic can be correct while the interpretation is wrong.
This page does not include tax rules, currency conversion, compound interest, statistical uncertainty, or a company-specific rounding policy. For a purchase, invoice, grade policy, or financial decision, compare the result with the original document and follow the applicable official definition.
Percentage calculator FAQ
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100. For example, 15% of 240 is 240 × 15 ÷ 100 = 36. Choose the X% of Y mode to check this calculation.
What percentage is one number of another?
Divide the part by the whole and multiply by 100. If 36 is compared with 240, the result is 36 ÷ 240 × 100 = 15%. The whole cannot be zero.
How do I calculate percentage increase or decrease?
Use (new value − old value) ÷ old value × 100. A positive result is an increase and a negative result is a decrease. The old value must not be zero.
What is the difference between percentage points and percent change?
Percentage points are the direct difference between two percentages. Percent change uses the earlier percentage as the base, so a move from 40% to 55% is 15 percentage points but a 37.5% relative increase.
Can this percentage calculator use decimals or negative values?
Yes. Decimal inputs are accepted, and negative values can be useful for signed data or changes. Check whether the result makes sense for your real-world quantity before using it.
Does the calculator save my values?
The calculation is performed by local JavaScript in your browser and does not require an account or a calculation API. Avoid entering confidential information on a shared device.
Quick summary
Choose the percentage question, enter the two values, and read both the answer and the formula. Use X% of Y for a part, X is what percentage of Y for a ratio, and percentage change for an old-versus-new comparison. Check the base number, watch for division by zero, and apply the rounding or policy rules that belong to your real-world data.