Four "amounts" get mixed up: mcg and mg measure mass; IU and syringe units measure completely different things.
Mass is a clean universal conversion: 1 mg = 1,000 mcg.
An International Unit (IU) measures biological activity, not mass — the mg equivalent is different for every substance.
A syringe unit is a volume mark (0.01 mL on U-100), not an amount of drug.
Convert everything to one mass unit first — mixing mg and mcg is where 10× errors happen.
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.
Watch
20 units is how many mg? Units measure liquid, not peptide
Peptide math: 2mg is how many units? The 30-second syringe answer
There are 1,000 micrograms (mcg) in 1 milligram (mg). So 0.25 mg = 250 mcg and 2.5 mg = 2,500 mcg.
Can I convert IU to mg directly?
Only per specific substance. An International Unit measures biological activity, and the IU-to-mg factor is defined separately for every compound by the WHO. There is no universal conversion.
How many units is 2.5 mg of tirzepatide?
It depends on the concentration you reconstituted to, because units measure volume, not mass. At 5 mg/mL, 2.5 mg is 0.5 mL = 50 units on a U-100 syringe; at 10 mg/mL the same 2.5 mg is 0.25 mL = 25 units. Work out your own vial in the tirzepatide calculator.
How many mg is 20 units of tirzepatide?
Again it depends on concentration. 20 units is 0.2 mL on a U-100 syringe; at 5 mg/mL that is 1 mg, and at 10 mg/mL it is 2 mg. To convert any unit count to mass, multiply your concentration (mg/mL) by the draw volume in mL (units ÷ 100).
References
World Health Organization Providing international biological reference standards — International Units are defined by biological activity and set per substance by the WHO Expert Committee on Biological Standardization (ECBS).WHO.int. 2023. WHO ECBS / International Standards
Zyra Track is a research and educational utility. Nothing on this page is medical advice, a dosing recommendation, or an endorsement of any compound. We never sell or source compounds and refuse sourcing questions. Consult a qualified clinician for decisions about your health.