Percent Basics and Conversions

A percent is simply a ratio that compares a number to 100. The word literally means "per hundred." Because they are ratios, every percent can be written as a fraction or a decimal. To turn a decimal into a percent, you move the decimal point two places to the right (multiplying by 100). To go from a fraction to a percent, you can divide the numerator by the denominator to get a decimal first. For example, $\frac{1}{2}$ is $0.50$, which is $50\%$. Think of it like a language: $0.75$, $\frac{3}{4}$, and $75\%$ are all saying the exact same thing in different ways.

You don't always need a calculator or a pencil. The 10% Rule is a mental math shortcut. To find $10\%$ of any number, just move the decimal point one place to the left. Once you have $10\%$, you can find $5\%$ (by taking half of the $10\%$ value) or $20\%$ (by doubling the $10\%$ value). This is how people calculate tips at restaurants in seconds! It's a great way to estimate if a price is right before you get to the checkout counter.