Definition: mg (milligram) and mcg (microgram) are units of weight. One milligram is a thousandth of a gram; one microgram is a millionth of a gram. So 1 mg = 1,000 mcg. Peptide vials are usually labeled in mg while single doses are often discussed in mcg, so you convert between them often.
How to convert
NIST defines the prefix milli (m) as one thousandth and micro (µ) as one millionth. The gap between them is a factor of 1,000.
mg to mcg: multiply by 1,000. mcg to mg: divide by 1,000.
| mg | mcg |
|---|
| 0.1 mg | 100 mcg |
| 0.25 mg | 250 mcg |
| 0.5 mg | 500 mcg |
| 1 mg | 1,000 mcg |
| 2.5 mg | 2,500 mcg |
| 5 mg | 5,000 mcg |
"mcg," "µg" and "ug" all mean microgram. The µ symbol is the SI symbol for micro; "ug" is what you get when a keyboard has no µ.
Worked example
A 250 mcg amount from a 5 mg vial in 2 mL
- Convert the vial: 5 mg = 5 × 1,000 = 5,000 mcg.
- Concentration: 5,000 mcg ÷ 2 mL = 2,500 mcg/mL.
- Volume for 250 mcg: 250 ÷ 2,500 = 0.1 mL.
- On a U-100 syringe, 0.1 mL is 10 units. Check: 10 × 0.01 mL × 2,500 mcg/mL = 250 mcg.
Keep the unit next to every number you log. "250" with no unit is either 250 mcg or 250 mg, and those differ 1,000 times.
Common mix-ups
- Moving the decimal the wrong way. 0.25 mg is 250 mcg, not 25 mcg and not 2,500 mcg.
- Mixing up mg and mL. mg is weight; mL is volume. They only line up when the concentration is exactly 1 mg/mL.
- Reading mcg as mg. The two differ by one letter and a factor of 1,000. FDA has also reported dosing errors from confusion between mL, mg and syringe units with compounded semaglutide.
Read more on Dose Track: Peptide reconstitution calculator · GLP-1 units to mg charts
General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
How many mcg are in 1 mg?+
There are 1,000 mcg in 1 mg. To go from mg to mcg, multiply by 1,000; to go back, divide by 1,000.
Is 250 mcg the same as 0.25 mg?+
Yes. 250 divided by 1,000 is 0.25.
Are mcg and µg the same?+
Yes. Both mean microgram, a millionth of a gram. µ is the SI symbol for micro.