An insulin syringe is marked in units rather than millilitres, which is convenient for insulin and mildly confusing for everything else. The good news is that the scale is fixed and the reading rules are simple once the barrel is understood.
What U-100 means
U-100 is a concentration standard for insulin: 100 units of insulin per millilitre. The syringe is graduated to match, so 100 units of barrel equals 1 mL of volume, and one unit equals 0.01 mL.
This is a volume scale. It does not know or care what is in the syringe. A reconstituted peptide drawn to 10 units occupies 0.1 mL, exactly as insulin drawn to 10 units would. What the 10 units represents in micrograms of peptide depends entirely on the concentration of the vial, which is a separate question covered in mg, mcg, mL and units.
The three common barrel sizes
| Barrel | Holds | Typical graduation | Smallest readable step |
|---|---|---|---|
| 0.3 mL | 30 units | 1 unit, often ½ unit | 0.005–0.01 mL |
| 0.5 mL | 50 units | 1 unit | 0.01 mL |
| 1 mL | 100 units | 2 units | 0.02 mL |
Every one of these is U-100. A 0.3 mL syringe is not a different scale; it is the same scale with less of it printed, spread over a similar physical length. That is the whole reason it reads more finely.
Why barrel size affects precision
Consider a 10 unit draw. On a 0.3 mL barrel that is a third of the way up a scale graduated every unit, and the mark is unambiguous. On a 1 mL barrel the same draw sits within the first tenth, against graduations every 2 units. Being one graduation out is a 2 unit error either way, but as a share of a 10 unit dose that is 20%.
Neither syringe is wrong. The dose is identical. What differs is how much reading error the barrel invites, which is a mechanical fact worth knowing when the choice comes up with your healthcare professional.
Reading the plunger correctly
- 1Hold the syringe at eye level
Reading from above or below introduces parallax, and on a barrel graduated every unit that is easily a unit of error.
- 2Find the front edge of the stopper
The rubber plunger stopper has a flat leading face and often a tapered cone in front of it. The measurement is taken at the flat face, the wide edge closest to the needle, not at the tip of the cone.
- 3Line the edge up with a graduation
The line should sit level with the flat face, not above or below it.
- 4Count from a labelled number
Barrels label every 5 or 10 units and rely on unlabelled ticks in between. Counting from the nearest printed number is more reliable than counting from zero.
Half-unit barrels
Some 0.3 mL syringes are graduated every half unit, which gives a readable step of 0.005 mL. For small draws that is a meaningful difference in resolution. The barrels are typically labelled as half-unit and are worth identifying deliberately rather than assuming, since they otherwise look much like a standard 0.3 mL.
Quick reference
U-100 volume equivalents.
| Units | mL | Fits in |
|---|---|---|
| 5 | 0.05 | Any barrel |
| 10 | 0.10 | Any barrel |
| 20 | 0.20 | Any barrel |
| 30 | 0.30 | 0.3 mL (full), 0.5 mL, 1 mL |
| 50 | 0.50 | 0.5 mL (full), 1 mL |
| 80 | 0.80 | 1 mL only |
To go the other way, from a prescribed dose in micrograms to a unit figure, the concentration of the vial is needed. The insulin syringe unit converter handles both directions.
