U-100 vs U-40 Syringe Units Explained
A “unit” on an insulin syringe is a volume mark. Which volume depends on the syringe's scale, and confusing the two scales is a 2.5-fold error.
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The two scales
Insulin syringes are printed for a specific insulin strength. U-100 syringes are made for insulin at 100 units per mL, so 100 marks span 1 mL and each mark-unit is 0.01 mL. U-40 syringes are made for 40 units per mL, so each unit is 0.025 mL. U-100 is the standard in the US for human insulin; U-40 is common in veterinary use.
For anything other than insulin, the “units” are just a volume scale. The mass in a unit depends entirely on the concentration of the solution, which is why the calculator asks for vial mg and water mL first.
Barrel sizes and their marks
The same scale comes in different barrel sizes. Smaller barrels are usually marked more finely.
Shown at half capacity: 50 units = 0.5 mL
Shown at half capacity: 25 units = 0.25 mL
Shown at half capacity: 15 units = 0.15 mL
Shown at half capacity: 20 units = 0.5 mL
Conversion table
| mL | U-100 units | U-40 units |
|---|---|---|
| 0.01 | 1 | 0.4 |
| 0.05 | 5 | 2 |
| 0.1 | 10 | 4 |
| 0.2 | 20 | 8 |
| 0.25 | 25 | 10 |
| 0.3 | 30 | 12 |
| 0.5 | 50 | 20 |
| 0.75 | 75 | 30 |
| 1 | 100 | 40 |
Reading small volumes
A 1 mL U-100 syringe is usually printed every 2 units, so a result of 5 units falls between marks. A 0.3 mL or 0.5 mL U-100 syringe is printed every unit (some 0.3 mL barrels every half unit). When a result is small, the calculator flags readings that fall between marks or below the smallest mark on the syringe you select.
Dead space, the liquid left in the needle and hub, is not counted on the scale. Low-dead-space syringes reduce it; the calculator's “measurements per vial” figure ignores it, so the real count is lower.
Syringe unit questions
How many units are in 1 mL?
100 on a U-100 syringe and 40 on a U-40 syringe. A unit is a graduation, not a mass; what it holds depends on the solution's concentration.
How many mL is 10 units?
0.1 mL on a U-100 syringe and 0.25 mL on a U-40 syringe.
Can I read a U-100 result on a U-40 syringe?
Not directly. Multiply U-100 units by 0.4 to get U-40 units (or divide U-40 units by 0.4 for U-100). Better: tell the calculator which syringe you have.