Core formula
Volume to draw (mL) = target dose ÷ concentration
If concentration is in mg/mL, target dose must be in mg. If concentration is in mcg/mL, target dose must be in mcg.
Unit conversions you must memorize
- 1 mg = 1000 mcg
- U-100 insulin syringe: 100 units = 1.0 mL
- Therefore: 1 unit = 0.01 mL
Worked example 1 (mg-based)
Given concentration = 5 mg/mL, target dose = 0.25 mg:
- mL to draw = 0.25 ÷ 5 = 0.05 mL
- On a U-100 syringe: 0.05 mL = 5 units
Worked example 2 (mcg-based)
Given concentration = 2000 mcg/mL, target dose = 300 mcg:
- mL to draw = 300 ÷ 2000 = 0.15 mL
- On a U-100 syringe: 0.15 mL = 15 units
Worked example 3 (from vial amount)
Vial contains 10 mg total, reconstituted with 2 mL total volume.
- Concentration = 10 ÷ 2 = 5 mg/mL
- Target 1 mg dose → 1 ÷ 5 = 0.2 mL → 20 units on U-100
Error-prevention checklist
- Write every step, including units, before drawing.
- Never mix mg and mcg in the same step without explicit conversion.
- Double-check concentration after any change in reconstitution volume.
- When possible, do an independent second check.
Common pitfalls
- Confusing syringe “units” with drug “units” (they are not universally interchangeable across medications).
- Using U-100 assumptions on a non-U-100 device.
- Rounding too aggressively for very small volumes.
Sources
- ISMP: Preventing medication errors with injectable products — https://www.ismp.org/recommendations
- FDA resources on insulin products and concentration labeling — https://www.fda.gov/drugs/drug-safety-and-availability/information-about-insulin-products
- CDC Injection Safety basics — https://www.cdc.gov/injection-safety/about/index.html