Peptide math trips people up because four different "amounts" are in play — and two of them (mcg and mg) measure mass while the other two (IU and syringe units) measure completely different things. In plain terms: not every "amount" is measuring the same kind of thing, so you can't convert blindly between them.
Mass: micrograms and milligrams
This one is a clean, universal conversion:
`` 1 mg = 1,000 mcg ``
| Milligrams | Micrograms |
|---|---|
0.25 mg | 250 mcg |
0.5 mg | 500 mcg |
1 mg | 1,000 mcg |
2.5 mg | 2,500 mcg |
Labels sometimes mix units — a vial in mg, a target in mcg — so converting to a single unit first prevents 10× errors.
International Units (IU)
An IU measures *biological activity*, not mass. In plain terms: it answers "how strong is the effect," not "how heavy is the powder." The conversion factor is defined separately for each substance by the World Health Organization's Expert Committee on Biological Standardization, which sets a reference preparation of known activity for each one1. There is no universal IU-to-mg rule. HCG, for instance, is dosed in IU because its activity — not its weight — is what's standardised. Always use the substance-specific factor from a reliable reference.
Syringe units
A syringe unit is a volume mark, not an amount of drug (see how to read an insulin syringe). On a U-100 syringe, 1 unit = 0.01 mL. How much compound sits in that 0.01 mL depends on your concentration.
The three kinds of "amount" at a glance
| Measurement | Measures | Converts universally? |
|---|---|---|
| mcg / mg | Mass | Yes — 1 mg = 1,000 mcg |
| IU | Biological activity | No — per substance |
| Syringe unit | Volume (0.01 mL on U-100) | No — depends on concentration |
Putting it together
To go from a dose to a draw:
- Convert the dose and the vial to the same mass unit (usually mcg).
- Compute concentration:
vial mcg ÷ water mL. - Draw volume:
dose ÷ concentration. - Units:
draw volume × 100(U-100).
The calculator does all four steps; this guide is so the numbers make sense.