Centimeter — Definition, Formula & Examples

A centimeter is a unit of length in the metric system equal to one hundredth of a meter. It is about the width of a large paperclip or your pinky fingernail.

A centimeter (abbreviated cm) is a metric unit of length defined as exactly $\frac{1}{100}$ of a meter, or equivalently, 10 millimeters.

Key Formula

1 \text{ cm} = \frac{1}{100} \text{ m} = 10 \text{ mm}

Where:

$\text{cm}$ = centimeters
$\text{m}$ = meters
$\text{mm}$ = millimeters

How It Works

You use centimeters to measure small and medium-sized objects like pencils, books, or your height. Most rulers show centimeters along one edge, with each small numbered line representing 1 cm. The tiny marks between centimeter lines are millimeters — there are 10 millimeters in every centimeter. To convert centimeters to meters, divide by 100. To convert meters to centimeters, multiply by 100.

Worked Example

Problem: A crayon is 9 cm long. How long is that in millimeters and in meters?

Convert to millimeters: Multiply the length in centimeters by 10, since there are 10 mm in 1 cm.

9 \text{ cm} \times 10 = 90 \text{ mm}

Convert to meters: Divide the length in centimeters by 100, since there are 100 cm in 1 m.

9 \text{ cm} \div 100 = 0.09 \text{ m}

Answer: The crayon is 90 mm or 0.09 m long.

Why It Matters

Centimeters are the go-to unit for everyday measuring in science class, art projects, and most countries around the world. Learning to measure in centimeters also builds a foundation for working with the entire metric system in later math and science courses.

Common Mistakes

Mistake: Confusing centimeters with inches when reading a ruler.

Correction: Most rulers have two sides — centimeters (smaller spaces, more marks) and inches (larger spaces). Check which side you are reading. One inch is about 2.54 cm, so centimeter marks are closer together.