The factor is 1000, and the direction is what people slip on
To go from micrograms to milligrams, move the decimal point three places left: 500 mcg is 0.5 mg. The full chain is 1 g = 1000 mg = 1,000,000 mcg, so each step is a factor of 1000.
Common vitamin and medication doses
| Substance | Typical dose | In mg |
|---|---|---|
| Folic acid (prenatal) | 400 mcg | 0.4 mg |
| Folic acid (pregnancy) | 600 mcg | 0.6 mg |
| Vitamin B12 (supplement) | 500–1000 mcg | 0.5–1.0 mg |
| Vitamin D3 | 25–50 mcg | 0.025–0.05 mg |
| Levothyroxine (thyroid) | 25–300 mcg | 0.025–0.3 mg |
| Iodine | 150 mcg | 0.15 mg |
Levothyroxine comes in steps as small as 12.5 mcg because the difference is clinically meaningful. This is exactly why these drugs are dosed in micrograms rather than milligrams.
The mcg vs mg mix-up is a safety issue, not a rounding one
- 1000× error. Reading 100 mcg as 100 mg is a dose a thousand times too large. For a drug like levothyroxine that can be dangerous.
- Why it says "mcg", not "µg". The Greek symbol µg can be misread as mg in handwriting, so the FDA, USP and ISMP require "mcg" on labels and prescriptions.
- IU is a different thing again. International units do not convert to mcg by a fixed number — it depends on the substance (1 IU of vitamin D is 0.025 mcg, so 1000 IU is 25 mcg).
Common questions
How many micrograms are in a milligram?
Exactly 1000 micrograms make one milligram. Micro means one millionth and milli means one thousandth, so they sit three orders of magnitude apart.
What is 400 mcg in mg?
400 mcg is 0.4 mg. That is the standard daily folic acid dose recommended for women who could become pregnant.
Is mcg the same as mg?
No, and treating them as equal is a dosing error. 1 mg is 1000 mcg, so mistaking one for the other means a dose 1000 times too high or too low.
