mg vs mcg Explained
The difference between milligrams and micrograms, how to convert between them, and the thousand-fold mistakes to avoid.
Last updated November 11, 2026
Educational use only
This site shows arithmetic and explains terminology. It never recommends doses, amounts or schedules, and it does not give medical advice. Any volume it shows is arithmetic on numbers you entered yourself. Follow the official product labeling and the instructions of a licensed healthcare professional or pharmacist. See the Medical Disclaimer.
The one fact to remember
1 mg = 1,000 mcg
A microgram (mcg or µg) is one thousandth of a milligram. The same mass is a number 1,000 times larger when written in mcg than in mg.
Converting
| Direction | Operation | Example |
|---|---|---|
| mg to mcg | × 1,000 | 0.25 mg = 250 mcg |
| mcg to mg | ÷ 1,000 | 750 mcg = 0.75 mg |
Why mistakes happen
- Different units on one page. A label may use mg while another document uses mcg.
- Decimals. 0.5 mg is 500 mcg, not 5 mcg.
- Unit dropped in notes. A bare “5” can mean 5 mg or 5 mcg, a thousand-fold difference.
Mass is not volume, and not IU
mg and mcg describe how much substance there is. mL and µL describe how much liquid there is. IU describes biological activity. They answer three different questions. See Understanding International Units.
How this connects to concentration
Concentration combines a mass unit with a volume unit, such as mg/mL or mcg/mL. Converting between them uses the same factor of 1,000: 1 mg/mL equals 1,000 mcg/mL. Use the mg to mcg converter to check your numbers.