Insulin syringe units explained
Written and maintained by the AVERON Research Desk · Last reviewed September 2026
Insulin syringes are widely used in the laboratory to measure small volumes accurately. Their markings are in "units", which causes a great deal of confusion, because a unit on a syringe is a measure of volume, not of how much compound it contains. This page explains the scale and the arithmetic. It is about measurement, not dosing.
What U-100 means
Most insulin syringes today are U-100. That means the scale is designed so that 100 units equals 1 mL. Therefore:
- 1 unit = 0.01 mL
- 10 units = 0.1 mL
- 50 units = 0.5 mL
- 100 units = 1 mL
A unit on a U-100 syringe always means the same volume, whatever liquid is inside it.
Common syringe sizes
| Syringe | Total volume | Units on the scale | Smallest marking (typical) |
|---|---|---|---|
| 1 mL | 1 mL | 100 | 1 or 2 units |
| 0.5 mL | 0.5 mL | 50 | 1 unit |
| 0.3 mL | 0.3 mL | 30 | 1 unit, sometimes half-units |
Smaller syringes spread the scale over a longer barrel, which makes small volumes easier to read precisely.
From units to milligrams
A unit tells you the volume. To know how much compound that volume contains, you need the concentration of the solution:
Amount in the syringe (mg) = concentration (mg/mL) × volume (mL)
and, on a U-100 syringe:
Volume (mL) = units ÷ 100
Worked examples
| Solution concentration | Units drawn | Volume | Amount of compound |
|---|---|---|---|
| 5 mg/mL | 10 units | 0.10 mL | 0.5 mg |
| 5 mg/mL | 20 units | 0.20 mL | 1 mg |
| 10 mg/mL | 10 units | 0.10 mL | 1 mg |
| 3.33 mg/mL | 12 units | 0.12 mL | 0.4 mg |
| 2 mg/mL | 25 units | 0.25 mL | 0.5 mg |
The same number of units contains a different amount of compound depending on how the vial was reconstituted. That is the central point of this page.
The mistakes that cause errors
Treating units as milligrams. A unit is a volume. "10 units" means 0.1 mL, and how many milligrams that holds depends entirely on the concentration.
Carrying numbers over from another product. A figure that worked for one vial, reconstituted with one volume, means something different for a vial reconstituted differently. Always recalculate for the solution actually in front of you.
Confusing syringe types. Older and some veterinary syringes use a U-40 scale, where 40 units equals 1 mL. Reading a U-40 syringe as if it were U-100 produces a large error. Check the marking on the packaging.
Confusing syringe units with international units (IU). They are unrelated. Syringe units measure volume; IU measure biological activity. See mg, mL and IU explained.
Not knowing the concentration. If the vial is not labelled with the volume it was reconstituted with, its concentration is unknown and it cannot be measured accurately.
Air bubbles
Air in the barrel takes up space that should be liquid, so a syringe with bubbles holds less than its scale suggests. Tapping the barrel so bubbles rise and expelling them before reading the scale gives an accurate measurement.