Percentage Calculator

This calculator answers the four percentage questions that come up most often: a share of a number, one number as a percentage of another, the change between two values, and adding or subtracting a percentage. Pick a mode, enter two values, and the result updates as you type.

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Percentage (X)

Number (Y)

Decimal places

Calculations keep full precision; only the displayed result is rounded.

15% of 240

Remaining amount: 204
As a decimal: 0.15

How to use this tool

1 Choose one of the four modes at the top of the workspace. Each mode shows only the fields it needs.
2 Type your numbers. A dot or a comma both work as the decimal separator, and negative values are accepted where they make sense.
3 Read the primary result, then check the secondary figures for the remainder, the decimal ratio, or the direction of the change.
4 Set the number of decimal places from 0 to 8. Calculations keep full precision internally; only the displayed value is rounded.
5 Use Copy result to copy the calculation and its answer, or Reset example to return to the sample values.

Formula and calculation logic

Percentage of a number

X% of Y = (X ÷ 100) × Y

Convert the percentage to a fraction by dividing by 100, then multiply by the base value. A rate above 100% returns a result larger than the base.

One number as a percentage of another

X is what % of Y = (X ÷ Y) × 100

Divide the part by the whole and multiply by 100. If the whole is 0 the ratio is undefined, so the calculator reports that instead of showing infinity.

Percentage change

Change % = ((New − Old) ÷ |Old|) × 100

The absolute value of the starting value keeps the sign of the result tied to the direction of the change. A rise from 80 to 100 is a 25% increase; a fall from 100 to 80 is a 20% decrease.

Adding or subtracting a percentage

Final = Base × (1 ± X ÷ 100); change = Base × X ÷ 100

The change amount is calculated first, then applied to the base. Adding 20% and then subtracting 20% does not return the original value, because the two percentages apply to different bases.

Worked example

What is 15% of 240?

A service costs 240 and you need the value of a 15% share.

15 ÷ 100 = 0.15
0.15 × 240 = 36

The answer is 36, with 204 remaining (240 − 36).

A price rises from 80 to 100

Compare the new price with the original price.

100 − 80 = 20
20 ÷ 80 = 0.25
0.25 × 100 = 25

The change is a 25% increase. Going back from 100 to 80 is a 20% decrease, even though the absolute difference is the same 20.

Add 20% to 120

Apply a 20% surcharge to a base amount of 120.

120 × 20 ÷ 100 = 24
120 + 24 = 144

The change amount is 24 and the final value is 144.

Edge cases and accuracy

Zero as the starting value

Percentage change divides by the starting value. When that value is 0 the change has no defined percentage, and the calculator says so instead of printing Infinity. The absolute difference is still shown.

Negative numbers

Negative percentages are valid in the "X% of Y" mode, where they simply reverse the sign of the result. In the change mode, direction is based on whether the new value is higher or lower, and the magnitude is measured against the absolute starting value.

Displayed precision

Values are stored as exact decimals and rounded only when displayed. Choosing 2 decimal places does not turn an internal 36 into 36.00, and repeated calculations do not accumulate rounding error.

Consecutive percentage changes

The add/subtract mode always applies the percentage to the amount in the base field. If you need two changes in a row, apply the first one, then use its result as the new base.

Frequently asked questions

How do I calculate X% of Y? +

Divide X by 100 and multiply by Y. For example, 15% of 240 is (15 ÷ 100) × 240 = 36. The "What is X% of Y" mode performs this calculation directly.

How do I find what percentage one number is of another? +

Divide the part by the whole and multiply by 100. 45 out of 180 is (45 ÷ 180) × 100 = 25%.

Why does the calculator show "undefined" for some percentage changes? +

Percentage change measures the difference relative to the starting value. If the starting value is 0 there is no baseline to divide by, so the result is mathematically undefined. The tool reports that instead of showing Infinity.

Is a 25% increase the same as a 20% decrease? +

No. The percentages are measured against different bases. Raising 80 to 100 is a 25% increase, while lowering 100 to 80 is a 20% decrease, even though the absolute difference is 20 in both cases.

Can I use negative percentages? +

Yes, in the "X% of Y" mode a negative percentage simply flips the sign of the result. For increases and decreases, enter a positive percentage and use the increase/decrease control so the direction stays explicit.

Does the calculator round intermediate values? +

No. Arithmetic uses exact decimal values and rounding happens only for display. This matters for money: 5% of 19.99 is exactly 0.9995, which displays as 1 with two decimal places but is not treated as 1 in the next step.

Which decimal separator should I use? +

Either a dot or a comma. Both are read as the decimal separator, and commas are not treated as thousands separators, so type 1234.56 rather than 1,234.56.