How to Read a U-100 Insulin Syringe: Units, mL, and Dose Math

A U-100 insulin syringe is the standard tool for subcutaneous peptide injections. But the markings can be confusing — especially when you are used to measuring in milligrams and the syringe is marked in "units."

The basics

U-100 means 100 units = 1 mL. Every line on the syringe represents a fraction of a millilitre.

Syringe size	Smallest line	Common use
0.3 mL (30 units)	0.5 units	Very small doses (< 0.3 mL)
0.5 mL (50 units)	1 unit	Most peptide doses
1.0 mL (100 units)	2 units	Larger volumes

Converting mg to units

You need to know two numbers:

1. Your peptide concentration after reconstitution (mg/mL)
2. Your desired dose (mg)

Example:

• Vial: 10 mg peptide + 2 mL BAC water = 5 mg/mL
• Desired dose: 0.5 mg
• Volume needed: 0.5 mg / 5 mg/mL = 0.1 mL = 10 units on a U-100 syringe

Common errors

• Using a U-40 syringe (for pet insulin) on a U-100 scale. This gives 2.5× the intended dose.
• Reading the wrong line — counting from 0 at the plunger tip (correct) vs at the top of the barrel.
• Confusing mg and mL. 1 mL is not the same as 1 mg.
• Not accounting for dead space in the needle hub (about 2–4 units).

Pro tips

• Always use the smallest syringe that fits your dose. A 30-unit syringe is easier to read precisely for small volumes.
• Draw to slightly past your mark, then push back to the exact line to eliminate air.
• When in doubt, use a syringe-unit converter or calculator.

TrackPep includes a built-in syringe unit converter that does the math for you — just enter your reconstitution and target dose, and it shows the exact units.