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milligram — Definition, Formula & Examples

A milligram is a metric unit of mass equal to one-thousandth of a gram. It is used to measure very small quantities, such as the amount of medicine in a tablet or vitamins in food.

A milligram (symbol: mg) is a unit of mass in the International System of Units (SI) defined as 10−310^{-3} grams, or equivalently 10−610^{-6} kilograms.

Key Formula

mass in mg=mass in g×1,000\text{mass in mg} = \text{mass in g} \times 1{,}000
Where:
  • mg\text{mg} = Mass in milligrams
  • g\text{g} = Mass in grams (1 g = 1,000 mg)

Worked Example

Problem: A vitamin C tablet contains 0.5 g of vitamin C. How many milligrams is that?
Set up the conversion: Since 1 gram equals 1,000 milligrams, multiply the number of grams by 1,000.
0.5 g×1,000=500 mg0.5 \text{ g} \times 1{,}000 = 500 \text{ mg}
Answer: The tablet contains 500 mg of vitamin C.

Why It Matters

Milligrams appear on every nutrition label and medicine bottle you read. Understanding this unit helps you compare doses, follow recipes that measure small ingredients, and interpret scientific data in biology and chemistry classes.

Common Mistakes

Mistake: Confusing milligrams (mg) with milliliters (mL).
Correction: Milligrams measure mass (how heavy something is), while milliliters measure volume (how much space a liquid takes up). They are not interchangeable.

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