Drug Calculations
Desired Over Have
Volume to administer = (Desired dose ÷ On hand dose) × Vehicle volume.
Weight-Based Dosing
Example: 70 kg patient, drug dosed at 5 mg/kg → 70 × 5 = 350 mg per dose.
Example: 25 kg child, drug dosed at 10 mg/kg/day in 2 divided doses → 25 × 10 = 250 mg/day ÷ 2 = 125 mg per dose.
BSA Dosing
Mosteller formula. Used for chemotherapy and some pediatric dosing when weight-based dosing is less accurate.
Concentration Calculations
Concentration = Amount of drug ÷ Volume of solution (e.g., mg/mL).
Dilution: C₁V₁ = C₂V₂ (concentration × volume before = concentration × volume after).
To find volume needed: Volume = Desired dose ÷ Concentration (mg/mL).
Unit Conversions for Medications
| Conversion | Factor |
|---|---|
| mcg to mg | ÷ 1000 |
| mg to g | ÷ 1000 |
| gr to mg | 1 gr = 60 mg |
| mL to L | ÷ 1000 |
| tsp to mL | 1 tsp = 5 mL |
| tbsp to mL | 1 tbsp = 15 mL |
Safe Dose Range
Calculate safe dose range using mg/kg/dose and mg/kg/day from drug references.
Per dose: Min dose = Weight (kg) × min mg/kg/dose. Max dose = Weight (kg) × max mg/kg/dose.
Per day: Min daily = Weight (kg) × min mg/kg/day. Max daily = Weight (kg) × max mg/kg/day. Divide by number of doses for per-dose range.
Always verify the ordered dose falls within the calculated safe range before administering.