What a "unit" actually is
Despite the name, an insulin unit on a syringe is a volume, not a mass. Specifically, every insulin syringe is calibrated so that:
1 insulin unit = 0.01 mL (one-hundredth of a milliliter)
The "U-100" label is a historical artifact from when insulin was standardized at 100 International Units per milliliter. To make dosing simpler for diabetics, manufacturers marked syringes in "units" — but the underlying mark is always just 0.01 mL.
For peptides, which are mass-based (mcg or mg), this means: units tell you how much liquid to draw, not how much drug. The actual peptide quantity per unit depends entirely on how you mixed the vial.
U-100 vs U-50 vs U-30
All three sizes use the same scale (1 unit = 0.01 mL). They differ only in total volume and tick granularity.
| Size | Total volume | Max units | Smallest tick | Best for |
|---|---|---|---|---|
| U-100 | 1 mL (cc) | 100 units | 2 units | Large doses, ≥ 50 units |
| U-50 | 0.5 mL | 50 units | 1 unit | Medium doses, 20–50 units |
| U-30 | 0.3 mL | 30 units | 0.5–1 unit | Small precise doses, ≤ 30 units |
The smaller the syringe, the easier it is to read small values — because the same "10 units" covers more physical length on a U-30 barrel than on a U-100.
How to read the ticks
Hold the syringe vertically with the needle pointing up. The plunger will have a dark rubber stopper — the top flat edge of the stopper (the one closest to the needle) is what counts as the volume line, not the tip or the back.
U-100 insulin syringe filled to the 30-unit mark. The top flat edge of the stopper is the reading point.
Common mistake: reading the tip of the stopper instead of the flat edge. The difference can be 2–4 units — meaningful when your entire dose is 10 units.
Converting units to mg and mcg
Since units are volume, converting to peptide mass requires two pieces of information:
- Total mg in the vial — printed on the label (e.g. 5 mg BPC-157)
- Volume of BAC water added — whatever you chose (e.g. 2 mL)
From those two numbers, concentration per unit is:
mcg per unit = (vial_mg × 1000) / (bac_ml × 100)
Example: a 5 mg vial reconstituted with 2 mL of bacteriostatic water:
(5 × 1000) / (2 × 100) = 5000 / 200 = 25 mcg per unit
So to hit a 250 mcg dose, draw 10 units.
If you don't want to do the math, use the free reconstitution calculator — it does it for you with every common ratio pre-filled.
Quick-reference table
Common vial sizes with 2 mL BAC water:
| Vial | BAC water | Concentration | mcg per unit |
|---|---|---|---|
| 2 mg | 2 mL | 1 mg/mL | 10 mcg |
| 5 mg | 2 mL | 2.5 mg/mL | 25 mcg |
| 10 mg | 2 mL | 5 mg/mL | 50 mcg |
| 15 mg | 3 mL | 5 mg/mL | 50 mcg |
| 20 mg | 2 mL | 10 mg/mL | 100 mcg |
Picking the right syringe size
Rule of thumb: use the smallest syringe your dose will fit in. You get finer tick marks and less physical room for misreading.
- Dose draws 5 units → U-30 (each unit is ~6 mm apart; very hard to misread)
- Dose draws 25 units → U-30 fits, but you're near the top — U-50 is more forgiving
- Dose draws 45 units → U-50
- Dose draws 80 units → U-100
Needle gauge matters less for peptides than for insulin. Standard peptide subcutaneous injections use 29G–31G, 5/16" (8 mm) length. Shorter and thinner is gentler; most people can't feel a 31G needle through abdominal fat.
Common mistakes
Confusing mL and units
"Draw 10 units" is not the same as "draw 1 mL." Ten units is 0.1 mL. Some research vials are labeled in mL only — check carefully which scale your syringe shows and which your source is quoting.
Using a U-100 for tiny doses
On a U-100, the tick spacing is coarser. Measuring 3 units on a U-100 barrel leaves a lot of room for eyeballing. If you regularly dose under 20 units, keep U-30 syringes on hand.
Reading the wrong part of the stopper
The top flat edge (closest to the needle) is the measurement line, not the rounded tip or the back edge. This is the #1 source of dosing error in peptide communities.
Mixing different vial ratios and forgetting
If you use 2 mL of BAC water for one vial and 3 mL for the next, "10 units" means two different doses. Track reconstitution ratios per vial — or better, log them in an app.
Reusing syringes
Insulin syringes are single-use. The needle dulls almost immediately on the first injection and becomes both more painful and more likely to cause micro-trauma. Used needles go into a sharps container, not a trash can.
Frequently asked
What does U-100 mean on a syringe?
U-100 means the syringe is calibrated for a concentration of 100 units per milliliter — so 1 unit = 0.01 mL. A U-100 syringe holds up to 1 mL total (100 units).
Are all insulin syringes U-100?
Yes — in the US and most international markets, all insulin syringes sold at pharmacies are U-100 calibrated. The difference between "U-100," "U-50," and "U-30" is the total volume of the syringe, not the unit scale.
Can I use an insulin syringe for subcutaneous testosterone?
Physically yes, but testosterone is oil-based and typically drawn with a thicker-gauge (23–25G) syringe. Insulin-syringe needles (29–31G) can be used for subq testosterone but take a long time to draw — and reusing them dulls the tip.
Do I need to sterilize the syringe?
No — insulin syringes ship sterile and are single-use. You do need to swab the vial top with an alcohol pad before drawing, and the injection site before injecting.