Milli- — Definition, Formula & Examples

Milli- is a metric prefix that means one-thousandth. When you see "milli-" in front of a unit, it tells you to divide that unit by 1,000.

The prefix milli- denotes a factor of $10^{-3}$ , or $\frac{1}{1{,}000}$ , of the base unit in the International System of Units (SI). Its symbol is **m**.

Key Formula

1 \text{ milli-unit} = \frac{1}{1{,}000} \text{ base unit} = 10^{-3} \text{ base unit}

Where:

$\text{milli-unit}$ = The smaller unit with the milli- prefix (e.g., milliliter)
$\text{base unit}$ = The standard metric unit (e.g., liter)

How It Works

Place "milli-" before any metric unit to create a unit that is 1,000 times smaller. For example, one millimeter is

\frac{1}{1{,}000}

of a meter, and one milligram is

\frac{1}{1{,}000}

of a gram. To convert from the base unit to milli-units, multiply by 1,000. To convert from milli-units back to the base unit, divide by 1,000.

Worked Example

Problem: A water bottle holds 2 liters. How many milliliters is that?

Recall the conversion: 1 liter equals 1,000 milliliters.

1 \text{ L} = 1{,}000 \text{ mL}

Multiply: Multiply the number of liters by 1,000.

2 \times 1{,}000 = 2{,}000

Answer: The water bottle holds 2,000 milliliters (mL).

Why It Matters

You encounter milli- units constantly in science and daily life — medicine doses are measured in milligrams, rainfall in millimeters, and drink volumes in milliliters. Understanding this prefix is essential for measurement tasks in science classes and health-related careers.

Common Mistakes

Mistake: Confusing milli- (one-thousandth) with centi- (one-hundredth).

Correction: Remember: milli- means

\frac{1}{1{,}000}

and uses the symbol "m," while centi- means

\frac{1}{100}

and uses the symbol "c." A millimeter is smaller than a centimeter.