Reading an
insulin syringe
The numbers on the barrel measure volume, not drug. Once that is clear the rest is straightforward — but it is the source of most of the confusion, because the same mark holds different amounts depending on how the vial was mixed.
What U-100 means
Insulin syringes are marked U-100, which fixes the scale on the barrel: 100 units to a millilitre. One unit is 0.01 mL. That relationship is a property of the syringe and never changes.
The name comes from U-100 insulin, which is formulated at 100 international units of insulin per millilitre so that the barrel could be labelled in insulin units directly. For insulin, the two meanings coincide. For anything else drawn into the same syringe they do not — the marks still measure hundredths of a millilitre, and carry no information about what is in the liquid.
A unit is a volume, not a dose
This is worth stating plainly, because "I take ten units" is a sentence that sounds complete and is not. Ten units is 0.1 mL. How much peptide that contains depends entirely on the concentration of the vial it came from:
| Vial | Solvent added | Concentration | 10 units contains |
|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 500 mcg |
| 5 mg | 2 mL | 2.5 mg/mL | 250 mcg |
| 5 mg | 5 mL | 1 mg/mL | 100 mcg |
| 10 mg | 2 mL | 5 mg/mL | 500 mcg |
Same syringe, same mark, a fivefold spread in what is actually in it. A unit figure is only meaningful alongside the concentration it was worked out from, which is why it is worth writing the concentration on the vial when you reconstitute it.
The three common sizes
All three are U-100. They differ in how much they hold and, as a result, in how finely they are graduated — a shorter barrel spreads fewer units over the same physical length, so each mark is easier to read.
| Capacity | Units | Typical graduation | Suits |
|---|---|---|---|
| 0.3 mL | 30 units | 1 unit, sometimes ½ | Small draws, where precision matters most |
| 0.5 mL | 50 units | 1 unit | Most reconstituted doses |
| 1 mL | 100 units | 2 units | Larger volumes, at the cost of resolution |
Graduation intervals vary between manufacturers, so read the barrel rather than assuming. The pattern holds regardless: the smaller the syringe, the finer the marks.
Choosing a size for a given draw
Pick the smallest syringe that comfortably holds the draw. A 12-unit dose measured on a 100-unit barrel sits between two marks near the very bottom, where a small misjudgement is a large fraction of the dose. The same 12 units on a 30-unit barrel is nearly half full and unambiguous.
If a draw comes out too small to read well, the fix is usually the vial rather than the syringe: less solvent gives a higher concentration and a bigger draw for the same dose. The reconstitution calculator shows the units for a given solvent volume, so it is quick to try a few and see which lands in a comfortable part of the barrel.
Reading the barrel accurately
- Read at the leading edge of the plunger seal, the flat front face, not the raised tip behind it.
- Look straight on. Reading a graduated barrel from an angle shifts the apparent level, the same parallax error as a measuring jug.
- Clear the air first. A bubble occupies volume on the scale but contains no liquid, so the draw is short by the size of the bubble.
- Check the scale printed on the barrel once per box. All three sizes above are U-100, but the number of units and the interval between marks differ.
Related
- Reconstitution calculator — converts a dose into units and shows it on the barrel
- mg, mcg, mL and units — every conversion, written out with worked examples
- TideTracker — syringe view, inventory and dose logging, kept on your iPhone
This page describes how a syringe is graduated. It does not recommend a dose, a compound or an injection technique, and it is not medical advice. Anything to do with what you take, or how, belongs with a qualified healthcare professional.