Foundations
Syringe units
What a "unit" on an insulin syringe actually measures, and how to translate between units and millilitres without a factor-of-ten error.
- Last updated:
- 2026-07-22
Overview
The "units" printed on a U-100 insulin syringe are hundredths of a millilitre. A syringe marked to 100 units holds 1.0 mL. 10 units is 0.1 mL. 5 units is 0.05 mL.
This is a volume scale, not a mass scale. A unit does not correspond to a fixed amount of peptide; the mass a unit represents is entirely determined by the concentration of the solution in the syringe.
Why it matters
Confusing units-as-volume with units-as-mass is one of the most common ways for an otherwise-correct calculation to end in a wildly wrong dose. Because insulin was historically dosed in unit-per-unit-of-hormone, the vocabulary invites the mistake.
Key concepts
U-100 convention
"U-100" refers to the syringe scale, not to the peptide inside it. On a U-100 syringe, 100 units is 1 mL regardless of what is in the syringe.
Translating units to mass
Multiply the concentration (mg/mL) by the volume in mL (units ÷ 100) to get mass in mg. For example: 5 mg/mL × (10 units ÷ 100) = 0.5 mg.
Different syringe scales exist
U-40 and U-500 syringes exist for legacy insulin formulations. On a U-40 syringe, 40 units is 1 mL. Never use a syringe of a different scale than the one your calculation assumed.
Common misconceptions
- "Take 10 units" is not a dose. It is a syringe reading, and it corresponds to whatever mass the current concentration puts in 0.1 mL.
- Insulin-syringe units are not universal peptide units. Reading "units" on a growth-hormone page and "units" on an insulin syringe are two different things.
- "Units" and "IU" (international units) are not the same word. IU is a defined biological-activity measurement for a specific hormone; syringe units are hundredths of a millilitre.
Practical interpretation
The Preparation Calculator on each compound page shows both the volume in mL and the corresponding U-100 syringe reading. Trust the calculator's arithmetic and cross-check its inputs, not its output.
Limitations
Not every peptide is administered in an insulin-syringe volume. Larger volumes (e.g. some intramuscular injections) use different syringes with mL-only markings.