How to calculate percentages, step by step

A percentage is a fraction out of $100$. So $15\%$ means $\tfrac{15}{100}$, or $0.15$ as a decimal. Turning the percent into a decimal is usually the first move.

Finding a percent of a number

To take a percent of a number, turn the percent into a decimal and multiply. Take $15\%$ of $80$.

What percent is one number of another

To find what percent $A$ is of $B$, divide $A$ by $B$ and multiply by $100$. Ask what percent $18$ is of $24$.

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Percent increase and decrease

For a percent change, divide the size of the change by the original amount, then multiply by $100$. A price rising from $40$ to $50$ changes by $10$.

Quick mental-math tips

• $10\%$ is just the number with the decimal point moved one place left: $10\%$ of $60$ is $6$.
• Find $5\%$ by halving $10\%$, and $20\%$ by doubling it.
• Percentages work both ways: $8\%$ of $50$ equals $50\%$ of $8$, which is $4$.

Decide which of the three types you have, convert to a decimal, and the rest is one multiplication or division.