A U-100 syringe is graduated in insulin units, where 100 units equal 1 millilitre. Converting milligrams to units depends on the concentration obtained on reconstitution, and that concentration is set by the volume of diluent added to the vial.
What does a U-100 syringe measure?
A U-100 syringe is graduated for insulin at a concentration of 100 units per millilitre. The scale measures volume: 100 units correspond to 1 mL, 50 units to 0.5 mL and 10 units to 0.1 mL.
From this follows the point behind most errors. The unit does not measure compound mass. A dose expressed in units only has meaning if the solution concentration is known, and that concentration depends on how much diluent was added to the vial.
How is concentration calculated after reconstitution?
Concentration results from dividing the vial milligrams by the millilitres of diluent added.
Concentration formula
- Concentration (mg/mL) = vial milligrams ÷ added volume (mL)
- 10 mg vial + 2 mL → 5 mg/mL
- 10 mg vial + 1 mL → 10 mg/mL
- 60 mg vial + 3 mL → 20 mg/mL
Lyophilised powder occupies negligible volume, so final volume is taken as equal to the added diluent for this calculation.
How is a dose in milligrams converted to units?
Once concentration is known, conversion is direct.
Conversion formula
- Units = (dose in mg ÷ concentration in mg/mL) × 100
- Equivalent: units = (dose mg × volume mL × 100) ÷ vial mg
Example with a 10 mg vial reconstituted with 2 mL, that is 5 mg/mL. A 0.25 mg dose equals 0.25 ÷ 5 = 0.05 mL, which on the U-100 scale corresponds to 5 units.
Conversion tables by concentration
The following equivalences cover common combinations. The unit figure changes with reconstitution volume even when the dose in milligrams is identical.
| Dose | 2 mg/mL | 5 mg/mL | 10 mg/mL | 20 mg/mL |
|---|---|---|---|---|
| 0.10 mg | 5 U | 2 U | 1 U | 0.5 U |
| 0.25 mg | 12.5 U | 5 U | 2.5 U | 1.25 U |
| 0.50 mg | 25 U | 10 U | 5 U | 2.5 U |
| 1.00 mg | 50 U | 20 U | 10 U | 5 U |
| 2.50 mg | 125 U | 50 U | 25 U | 12.5 U |
| 5.00 mg | — | 100 U | 50 U | 25 U |
Cells marked with a dash exceed the capacity of a 1 mL U-100 syringe and would require more than one administration or a different concentration.
Which reconstitution volume is preferable?
There is no universally correct volume, but the selection criterion is objective. It is a trade-off between measurement precision and injected volume.
- ✓Larger volume, lower concentration: each dose occupies more units on the scale, reducing relative error when measuring small doses.
- ✓Smaller volume, higher concentration: the volume administered is smaller, but a one-unit error represents a larger fraction of the dose.
- ✓Practical rule: concentrations placing the usual dose between 10 and 50 units allow comfortable reading of the scale.
The site's reconstitution calculator automates these conversions for any combination of vial, volume and dose.
Frequent conversion errors
Four common confusions
- Treating units as milligrams. They are different quantities: the unit measures volume on an insulin scale.
- Transferring a unit figure between different concentrations. The same dose in milligrams equals different unit numbers depending on reconstitution volume.
- Omitting concentration when recording a dose. A record saying "10 units" without stating concentration is irreproducible.
- Assuming powder volume alters the calculation. Its contribution is negligible against the diluent volume.
Which diluent is used?
Bacteriostatic water contains 0.9% benzyl alcohol as a bacteriostatic agent, allowing repeated withdrawals from the same vial over a limited period. Sterile water for injection contains no preservative and is intended for single withdrawal.
Diluent is added by letting it run down the inner wall of the vial, not directly onto the powder. Dissolution is completed by gentle rotation. Vigorous shaking generates foam and promotes denaturation through air-liquid interface stress, a mechanism described in the protein stability literature.(1)
References
- Manning MC, Chou DK, Murphy BM, et al. Stability of protein pharmaceuticals: an update. Pharm Res. 2010;27(4):544-575. DOI: 10.1007/s11095-009-0045-6.
- Wang W. Lyophilization and development of solid protein pharmaceuticals. Int J Pharm. 2000;203(1-2):1-60. DOI: 10.1016/s0378-5173(00)00423-3.
