mg, mL, mcg and IU explained
Written and maintained by the AVERON Research Desk · Last reviewed September 2026
Research peptides are described in several different units, often on the same label: milligrams on the vial, millilitres of water, units on a syringe, sometimes international units. They measure fundamentally different things, and most measuring mistakes come from treating one as if it were another.
The four you will meet
mg (milligram) — a unit of mass. It tells you how much compound there is. A vial labelled 10 mg contains 10 milligrams of the compound.
mcg (microgram) — also a unit of mass, one thousandth of a milligram. Sometimes written µg.
- 1 mg = 1,000 mcg
- 0.5 mg = 500 mcg
- 0.25 mg = 250 mcg
mL (millilitre) — a unit of volume. It tells you how much liquid there is. It says nothing on its own about how much compound that liquid contains.
IU (international unit) — a unit of biological activity, not mass or volume. Each IU is defined separately for each substance by an international reference standard.
Concentration links mass and volume
Mass and volume are connected only through concentration:
Concentration (mg/mL) = mass (mg) ÷ volume (mL)
Dissolve 10 mg in 2 mL and you have 5 mg/mL. Every 0.1 mL of that solution contains 0.5 mg. Dissolve the same 10 mg in 5 mL and every 0.1 mL contains only 0.2 mg. The mass in the vial did not change; the concentration did.
Why IU cannot be converted with one formula
Because an IU measures activity, there is no universal conversion between IU and milligrams. The conversion depends entirely on the substance. For somatropin (human growth hormone), for example, 1 mg is commonly equated to about 3 IU. That figure applies only to somatropin. Applying it to any other compound would be meaningless.
Most research peptides are specified by mass, in mg or mcg, rather than in IU.
Syringe units are not IU
The "units" printed on an insulin syringe are a volume scale: on a U-100 syringe, 1 unit is 0.01 mL. They have nothing to do with international units, even though both are called units. See insulin syringe units explained.
Quick reference
| Unit | Measures | Example |
|---|---|---|
| mg | Mass | 10 mg in a vial |
| mcg | Mass (1/1000 mg) | 250 mcg = 0.25 mg |
| mL | Volume | 2 mL of water added |
| mg/mL | Concentration | 10 mg ÷ 2 mL = 5 mg/mL |
| Syringe unit (U-100) | Volume | 1 unit = 0.01 mL |
| IU | Biological activity | Substance-specific, no universal conversion |
The mistakes to avoid
- Confusing mg and mcg. A thousandfold difference. Always check which one a figure uses.
- Treating mL as an amount of compound. It is only an amount of liquid.
- Treating syringe units as IU. They measure different things entirely.
- Applying one compound's IU conversion to another. IU conversions are never transferable.
Frequently asked questions
Sources
- World Health Organization. WHO International Standards for biological substances. WHO. Link