Millimeter (Millimetre) — Definition, Formula & Examples

A millimeter (mm) is a very small unit of length in the metric system, equal to one-thousandth of a meter. It is about the thickness of a dime or a paperclip wire.

A millimetre (symbol: mm) is a unit of length in the International System of Units (SI) defined as exactly $\frac{1}{1000}$ of a metre, or equivalently $\frac{1}{10}$ of a centimetre.

Key Formula

1 \text{ mm} = \frac{1}{10} \text{ cm} = \frac{1}{1000} \text{ m}

Where:

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

How It Works

You use millimeters when you need to measure something very small or need extra precision. On a standard ruler, the tiny lines between each centimeter mark represent individual millimeters — there are 10 of them per centimeter. To convert centimeters to millimeters, multiply by 10. To convert millimeters to centimeters, divide by 10. To convert meters to millimeters, multiply by 1,000.

Worked Example

Problem: A pencil is 17.5 cm long. How many millimeters is that?

Recall the conversion: There are 10 millimeters in every centimeter.

1 \text{ cm} = 10 \text{ mm}

Multiply: Multiply the length in centimeters by 10.

17.5 \times 10 = 175 \text{ mm}

Answer: The pencil is 175 mm long.

Why It Matters

Millimeters show up whenever precision matters — in science experiments, engineering drawings, and medical dosing. Learning to read and convert millimeters builds a foundation for measurement skills used throughout math and science courses.

Common Mistakes

Mistake: Confusing millimeters with centimeters when reading a ruler.

Correction: Remember that the small lines on a ruler are millimeters and the numbered lines are centimeters. There are exactly 10 mm in 1 cm, so millimeters are the smaller unit.