Insulin syringe units (U-100)

A U-100 insulin syringe is marked in units rather than millilitres. The scale was designed for insulin at 100 international units per millilitre, so one unit of volume is 0.01 mL and the full 1 mL syringe reads 100 units. Smaller syringes, 0.5 mL and 0.3 mL, use the same unit but with finer markings. Peptide handling borrows the syringe and the scale because the needle is short and fine and the graduations are easy to read.

The unit measures volume only. How much peptide a unit contains depends entirely on the concentration set at reconstitution. A 5 mg vial mixed with 2 mL gives 2.5 mg/mL, so one unit (0.01 mL) holds 25 µg and a 250 µg draw is ten units. Mix the same vial with 1 mL and each unit holds 50 µg. This is the most common source of error in research handling, which is why the dosage calculator asks for vial size, diluent volume and target amount and returns the unit mark to draw.

Note that these syringe units have nothing to do with the International Units used to measure biological activity of products such as HCG, which are a measure of potency, not volume. The dosing-math guide keeps the two apart.