Milligrams and grams
The milligram is one thousandth of a gram. Medicine doses, vitamins and nutrients such as sodium and caffeine are labeled in milligrams.
The gram is one thousandth of a kilogram. Recipes, nutrition labels, postage and small parcels use grams.
Worked example
Convert 500 mg: 500 × 0.001 = 0.5, so 500 milligrams is 0.5 grams. Likewise 100 mg = 0.1 g.
Estimating in your head
The factor is a power of ten, so just move the decimal point 3 places to the left: 500 mg = 0.5 g.
Going the other way
To convert grams to milligrams, divide by 0.001 (or multiply by 1,000). The swap button between the boxes opens that page.
How this works
g = mg × 0.001
1 mg = 0.000001 kg and 1 g = 0.001 kg, both by definition, so the factor is their ratio: 0.001 exactly. Results drop binary floating-point noise beyond 15 significant digits and display at the precision you choose (6 significant figures by default). Copy result copies the full value.
Frequently asked
How many grams are in a milligram?
1 milligram = 0.001 grams, exactly. The value follows from the unit definitions, so it never changes.
How do I convert milligrams to grams?
Multiply by 0.001. For example, 100 mg × 0.001 = 0.1 g.
What is 1,000 mg in grams?
1,000 mg = 1 g. The table next to the converter lists more common values.
Is the result rounded?
Only on screen. The page computes with the exact definitions and removes floating-point noise past 15 significant digits; pick 3 to 12 significant figures, or use Copy result for the full value.
Sources
- NIST SP 811 Appendix B.8 — Factors for units listed alphabetically
- Federal Register 59-5442 (1959) — Refinement of values for the yard and the pound
- Wikipedia — Avoirdupois system
Reviewed Oct 17, 2026 · test vectors fixtures/units/mass/mg-to-grams/