Percentage Calculator

Find what percentage one number is of another.

Formula

How the calculation works

Finding a percentage of a number is one multiplication: convert the percentage to a decimal and multiply. 15% of 80 is 0.15 times 80, which is 12. The conversion is the only step people fumble, because 15% is 0.15 and not 1.5 -- a decimal point in the wrong place is a tenfold error, and it is the single most common mistake with percentages.

Percentages are reversible in a useful way. 8% of 50 equals 50% of 8, because both are 4. The general rule is that a% of b always equals b% of a. When one direction is easier to do in your head -- 4% of 75 is awkward, 75% of 4 is 3 -- swap them rather than reaching for a calculator.

The word 'percent' means 'per hundred', so a percentage is a fraction whose denominator is fixed at 100. That is why percentages compare cleanly across different sizes: 12 out of 80 and 30 out of 200 are both 15%, even though the raw counts differ. Use percentages to compare rates and raw numbers to compare amounts.

Common mistakes

• Moving the decimal point one place instead of two, turning 15% into 1.5 and inflating the answer tenfold.
• Adding a percentage to a number by adding the percentage number itself: 80 plus 15% is 92, not 95.
• Comparing percentages without checking the base. A rise from 2 to 3 is 50%, from 200 to 300 is also 50%, but one is a rounding difference and the other is a budget change.

When to use it

• Use it whenever you need one number's share of another: a tax line, a commission, a pay component, a discount.
• For the change between two numbers rather than a share of one, use the percent-change tool -- the arithmetic looks similar and is not the same.

Worked example

45 out of 180 is 25%.

Common questions

Increase or decrease — which?

This gives the ratio only. Growth needs both the before and after numbers, which is a separate tool.

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Last updated: 2026-09-29