U-100 syringes show volume, not mass
The "units" on a U-100 insulin syringe are a graduation of volume, scaled for insulin at 100 units per mL — so 1 unit = 0.01 mL. The syringe has no idea what's in the liquid. Converting between syringe units and peptide milligrams requires one extra number: the concentration.
Why "U-100" means what it means
U-100 refers to insulin's standard concentration: 100 units of insulin biological activity per 1 mL of solution. Syringes were standardized to match: 100 marked "units" per 1 mL barrel volume. The relationship is fixed in glass and steel, regardless of what's actually in the syringe.
The result is that "units" became a convenient volume shorthand. For an insulin user, units of insulin and units on the syringe coincide perfectly. For a peptide user with a different concentration, they do not.
The two-step conversion
- Volume needed (mL) = Target dose (mg) ÷ Concentration (mg/mL)
- Units to draw = Volume needed (mL) × 100
Worked examples
| Peptide | Vial setup | Concentration | Target dose | Volume | Units on syringe |
|---|---|---|---|---|---|
| BPC-157 | 5 mg + 2 mL BAC water | 2.5 mg/mL | 250 mcg (0.25 mg) | 0.1 mL | 10 units |
| BPC-157 | 5 mg + 5 mL BAC water | 1 mg/mL | 250 mcg (0.25 mg) | 0.25 mL | 25 units |
| Semaglutide (compounded) | 10 mg + 2 mL BAC water | 5 mg/mL | 0.25 mg | 0.05 mL | 5 units |
| Tirzepatide (compounded) | 20 mg + 2 mL BAC water | 10 mg/mL | 2.5 mg | 0.25 mL | 25 units |
| TB-500 | 5 mg + 2 mL BAC water | 2.5 mg/mL | 2 mg | 0.8 mL | 80 units |
The same dose, different units
Notice that the same 0.25 mg BPC-157 dose appears as "10 units" with one reconstitution and "25 units" with another. The dose is identical; only the volume drawn differs. This is the core point: the unit number is a property of how you mixed the vial, not the dose.
U-100 vs U-40 vs U-500
Other insulin concentrations exist and use their own scaled syringes:
- U-40 syringes (mostly veterinary): 40 units per mL → 1 unit = 0.025 mL.
- U-500 syringes (concentrated insulin): 500 units per mL → 1 unit = 0.002 mL.
If you accidentally use a U-40 syringe with a U-100 dose, you draw 2.5× the intended volume. See the dedicated post on U-500 insulin and special syringes for the prescription-only flip side.
Why this matters more for peptides than insulin
Insulin doses are typically prescribed and dialed in units that match the syringe's scale. Peptide doses are typically expressed in mg or mcg, and the conversion to units depends on every individual's vial. The mental model "1 unit ≈ 1 mg" is wrong by a factor of 10–25× for most peptides, and the math habit is what prevents the most common dosing errors.
The reconstitution calculator does this conversion automatically — enter vial mass, diluent volume, target dose, and it returns both the mL volume and the U-100 unit count.
FAQ
My syringe says "1 mL" not units. Can I still use it?
Yes, but you need a precise volume measurement (typically 0.01 mL graduations) and the math returns mL directly without the second step. For doses under 0.3 mL, a U-100 syringe with units is more precise.
What if I miscounted units and drew 12 instead of 10?
You injected 20% more peptide than intended. For most peptides this is well within safety margins but enough to cause noticeable extra side effects. For semaglutide/tirzepatide titration doses, the cumulative effect of repeated 20% overshoots is meaningful — fix the next dose, don't compensate-double-down.
Does the unit scale change with the needle size?
No. Needle gauge (the bore) and needle length don't affect the unit graduation on the barrel. A 31G ½" and a 29G ⅝" U-100 syringe both have the same scale.
Why are some peptide vials labeled in "iu" or "IU"?
Growth-hormone analogs (and a few legacy compounds) carry an IU label — International Units, a biological-activity unit, not a U-100-syringe unit. The two scales coincide for insulin only. For peptides, check the package insert.
Related reading
Compute every draw before you uncap
Peptide Protocol's calculator does the mg-to-units conversion for each vial you log, so you never recompute by hand.
Get the iPhone app →Informational and educational only. Not medical advice. Consult a licensed clinician before starting, changing, or stopping any peptide protocol. Mentions of investigational, compounded, or research-use peptides are for informational purposes; many such substances are not FDA-approved for human use.