Definition: A U-100 syringe is an insulin syringe marked for insulin that holds 100 units per mL. Each unit mark equals 0.01 mL, so 100 marks fill 1 mL. It is the syringe made for human U-100 insulin, and the one most peptide dose math assumes.
Sizes and what the marks mean
The Humulin R U-100 carton says "use only with a U-100 syringe," because the insulin is 100 units/mL and the syringe is marked to match. Insulin syringe barrels come in sizes such as 0.3 mL, 0.5 mL and 1 mL (the embecta range, for example), and embecta offers a half-unit scale on the 0.3 mL barrel.
U-100 barrel sizes. Capacity = barrel volume × 100.
| Barrel | Holds | One mark |
|---|
| 0.3 mL | 30 units | 0.01 mL |
| 0.5 mL | 50 units | 0.01 mL |
| 1 mL | 100 units | 0.01 mL |
Worked example with a peptide vial
With a peptide, the marks still measure volume. What each mark holds depends on how the vial was mixed.
5 mg vial in 2 mL, drawn to 25 units
- 25 marks × 0.01 mL = 0.25 mL.
- Vial concentration: 5 mg ÷ 2 mL = 2.5 mg/mL.
- Amount: 0.25 mL × 2.5 mg/mL = 0.625 mg (625 mcg).
- Check: each mark holds 25 mcg, and 25 × 25 = 625 mcg.
A smaller barrel has more space between marks, which makes small volumes easier to read. Pick the barrel by the volume you draw, not the dose number.
Common mix-ups
- Using a U-40 syringe by mistake. U-40 marks are 0.025 mL each, 2.5 times larger. See U-40 vs U-100.
- Treating units as mg. 10 units is 0.1 mL, which can be any amount of drug depending on concentration.
- Oversized syringes. FDA has reported compounded semaglutide errors where patients were given syringes far larger than the dose volume.
Read more on Dose Track: U-40 vs U-100 insulin syringes · GLP-1 units to mg charts · Peptide reconstitution calculator
General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
How many mL is 1 unit on a U-100 syringe?+
One unit mark is 0.01 mL. 10 units is 0.1 mL and 100 units is 1 mL.
How many units does a 0.5 mL U-100 syringe hold?+
50 units, because 0.5 mL times 100 units per mL is 50.
Can a U-100 syringe be used for peptides?+
People often use one to measure peptide volumes. The marks measure volume, so you must know the vial concentration to know what dose a mark holds.