There are 1,000 milligrams in a gram. The prefix milli means one thousandth, so 1 mg = 0.001 g. Multiply grams by 1,000 to get milligrams, and divide milligrams by 1,000 to get grams: 2.5 g is 2,500 mg, and 350 mg is 0.35 g. A kilogram holds 1,000,000 mg.
How many milligrams are in a gram?
The International System of Units builds larger and smaller units from a base unit by adding a prefix. NIST's prefix list gives milli (m) as 10^-3, kilo (k) as 10^3 and micro (μ) as 10^-6. The gram is 1/1,000 of the base unit, the kilogram, so one gram is 1,000 mg, and a milligram is a thousandth of a gram.
NIST adds a detail for mass: because the kilogram already carries the prefix kilo, its multiples and submultiples are formed from the gram, so the correct form is 1 mg, not 1 μkg.
Formulas
milligrams = grams × 1,000
grams = milligrams ÷ 1,000
grams = kilograms × 1,000
micrograms = milligrams × 1,000
The converters grams to mg, grams to kg and mg to mcg take any value.
Metric prefix chart
| Prefix | Symbol | Factor | Mass example | Length example | Volume example |
|---|---|---|---|---|---|
| kilo | k | 1,000 | 1 kg = 1,000 g | 1 km = 1,000 m | 1 kL = 1,000 L |
| (base) | 1 | 1 g | 1 m | 1 L | |
| deci | d | 0.1 | 1 dg = 0.1 g | 1 dm = 0.1 m | 1 dL = 0.1 L |
| centi | c | 0.01 | 1 cg = 0.01 g | 1 cm = 0.01 m | 1 cL = 0.01 L |
| milli | m | 0.001 | 1 mg = 0.001 g | 1 mm = 0.001 m | 1 mL = 0.001 L |
| micro | μ | 0.000001 | 1 μg = 0.000001 g | 1 μm = 0.000001 m | 1 μL = 0.000001 L |
Moving one step between kilo, base and milli multiplies or divides by 1,000, while centi is a factor of 100. That gives 1 m = 100 cm = 1,000 mm, and 1 cm = 10 mm. See the converters cm to mm, liters to ml and ml to liters.
Grams to milligrams chart
| Grams | Milligrams |
|---|---|
| 0.001 g | 1 mg |
| 0.01 g | 10 mg |
| 0.1 g | 100 mg |
| 0.25 g | 250 mg |
| 0.5 g | 500 mg |
| 1 g | 1,000 mg |
| 2 g | 2,000 mg |
| 5 g | 5,000 mg |
| 10 g | 10,000 mg |
| 100 g | 100,000 mg |
| 1 kg (1,000 g) | 1,000,000 mg |
Worked examples
Example 1: a food label
A label lists 480 mg of sodium per serving. 480 ÷ 1,000 = 0.48 g per serving, so 5 servings contain 5 × 480 = 2,400 mg, or 2.4 g.
Example 2: a supplement with grams on the bottle
A 1.5 g powder serving is 1.5 × 1,000 = 1,500 mg. A 250 mg capsule would take 6 capsules to reach the same amount (1,500 ÷ 250 = 6).
Example 3: milligrams to micrograms
A vitamin listed at 0.5 mg is 0.5 × 1,000 = 500 μg. Reading "500 mcg" as 500 mg would be an error by a factor of 1,000.
Example 4: the same idea for volume
A 750 mL bottle is 750 ÷ 1,000 = 0.75 L. A 2.5 L bottle is 2,500 mL. The rule is identical because milli is milli: it always means a thousandth.
Example 5: the same idea for length
A 2.4 m board is 2.4 × 100 = 240 cm or 2.4 × 1,000 = 2,400 mm. A 35 mm screw is 3.5 cm and 0.035 m. To mix in imperial, 25.4 mm is 1 inch; see mm to inches.
Common mistakes
- Mixing mg and mcg. One milligram is 1,000 micrograms, and mcg or μg is the smaller unit. Check the symbol before comparing amounts.
- Treating g as kg. 1 kg is 1,000 g, not 100 g. The factor of 1,000 applies to kilo, milli and base units; only centi (a hundredth) uses 100.
- Confusing mass and volume. A milliliter is a volume and a milligram is a mass. Water is close to 1 g per mL, but many liquids are not; the cups to grams guide shows why.
- Mixing systems. For ounces and pounds see how many grams in a pound and the metric vs imperial guide.
This page explains unit conversion for general reference; it is educational and not medical or dosing advice.