Short answer: On a U-100 insulin syringe, "units" are a volume mark, not a drug amount: 100 units equals 1 mL, so each unit is 0.01 mL. A 1 mL syringe holds 100 units, a 0.5 mL syringe 50 and a 0.3 mL syringe 30. To turn units into milligrams of peptide, you need the vial's concentration: units ÷ 100 × mg per mL. Read the dose at the edge of the black rubber plunger closest to the needle, check whether your barrel has 1-unit or 2-unit lines, and remember that very small volumes are measured less accurately.
Most peptides are drawn with insulin syringes, even though they are not insulin. That works because the syringe is really just a measuring tool for volume. But the markings say "units," and that word causes more mistakes than anything else in peptide prep. This guide covers how to physically read the syringe. For the full reconstitution math, see our reconstitution units guide or the reconstitution calculator.
The "U-100" on the package refers to insulin strength. FDA insulin labels describe standard insulin as "100 units/mL (U-100)." A U-100 syringe is marked so that, when it is filled with U-100 insulin, the number you read is the number of insulin units. Strip insulin out of it and the syringe is simply a volume scale:
So when someone says "draw 20 units of BPC," they mean 0.20 mL of whatever solution is in the vial. How much peptide that is depends entirely on how the vial was mixed.
Insulin labels take syringe mix-ups seriously. The Humulin R U-500 label warns that U-500 insulin "contains 5 times as much insulin (500 units/mL) in 1 mL" as U-100, and that using a U-100 syringe for a U-500 dose "can give you a 5 times overdose." The same logic applies to U-40 syringes, which are marked for a different strength; see U-40 vs U-100 syringes.
| Barrel | Holds | Typical line spacing | Best for |
|---|---|---|---|
| 0.3 mL | 30 units | 1 unit per line; some have half-unit lines | Small volumes, under about 30 units |
| 0.5 mL | 50 units | 1 unit per line | Volumes up to 50 units |
| 1 mL | 100 units | Usually 2 units per line | Larger volumes |
Line spacing varies by brand, so check the printed numbers on your own barrel. Count the small lines between two numbered lines (for example, between 10 and 20). Ten small spaces means each line is 1 unit. Five means each line is 2 units. Getting this wrong on a 1 mL syringe doubles or halves your volume.
You need one number from the vial: its concentration, in mg per mL. If you mixed the vial yourself:
Concentration (mg/mL) = peptide in the vial (mg) ÷ water added (mL)
Then:
mg in the syringe = (units ÷ 100) × concentration (mg/mL)
And the reverse, if you know the amount and want the line to draw to:
units = (amount in mg ÷ concentration in mg/mL) × 100
Many peptides are discussed in micrograms (mcg). 1 mg = 1,000 mcg.
These examples show how the arithmetic works. The amounts are illustrations, not dose recommendations.
The same "10 units" means a different amount in each vial. This is why you should never copy someone else's units without knowing their concentration.
Five units is a tiny volume. That leads to the next point.
The smaller the volume, the more a small misreading matters. A study of children with type 1 diabetes and their parents measured how accurately they drew up doses (Lteif 1999). For regular insulin doses under 5 units, the average absolute error with syringes was 9.9%, compared with 3.2% for doses over 5 units. Pens were more accurate at low doses.
The practical takeaway: if your planned draw is only a few units, you are working in the range where syringe errors are largest. A 0.3 mL barrel with finer lines makes small volumes easier to read than a 1 mL barrel. The concentration you mix also changes how many units a given amount takes up, which is a decision to make with your prescriber or pharmacist, not by trial and error.
The safest habit is to do the concentration math once per vial and save it. In Dose Track, you record each vial's peptide amount and water volume when you reconstitute it; the app works out the concentration and the units to draw for a dose, and subtracts each logged dose from the vial's remaining volume. You still read the syringe yourself, but you are not redoing arithmetic at 6 a.m. For storage once mixed, see how long reconstituted peptides last and bacteriostatic vs sterile water.
Questions about the app? See the help center or join the Dose Track community.
Every number in this article comes from one of these sources, checked on October 22, 2026. This article is for education. It is not medical or legal advice, and nothing here is a dose recommendation.