Guide
mg, mcg, IU and Syringe Units, Explained
Four words that look alike and measure different things.
Four different measurements
| Term | Measures | Conversion |
|---|---|---|
| mg (milligram) | Mass | 1 mg = 1,000 mcg |
| mcg or µg (microgram) | Mass | 1 mcg = 0.001 mg |
| IU (international unit) | Biological activity, defined per substance | No universal conversion; somatropin: 1 mg = 3 IU |
| Syringe unit (U-100) | Volume | 1 unit = 0.01 mL |
mg and mcg
Both measure mass. A microgram is a thousandth of a milligram. Most peptide vials are labelled in mg; many small amounts are written in mcg. Use the mcg to mg converter when in doubt.
International units
An IU is defined separately for each substance, based on its biological effect. There is no general rule for turning IU into mg; each substance has its own conversion, and many have none. HCG and somatropin are the common IU-labelled products. Use the IU calculator for them.
Syringe units
The "units" on an insulin syringe are a volume, not an amount of drug. On a U-100 syringe, one unit is 0.01 mL. How much peptide is in that volume depends on the concentration. This is the most common source of confusion: "10 units" of a peptide says nothing about the mg until you know the concentration.
A quick check that catches most errors
Before drawing anything, ask: is my dose smaller than the whole vial? A 5 mg vial cannot contain a 250 mg dose; the dose was almost certainly meant to be 250 mcg. Every calculator here refuses a dose larger than the vial for this reason.
Frequently asked questions
Is 1 unit the same as 1 mg?
No. A syringe unit is a volume (0.01 mL on a U-100 syringe). How many mg it holds depends on the concentration.
How many mcg is 0.1 mg?
0.1 mg is 100 mcg.