Drug Calculations

Quick-reference material for licensed nurses and students. Values are drawn from commonly cited clinical guidelines and nursing references — always verify against your facility protocols. About our editorial standards · Report a correction

Desired Over Have

Dose = (Desired / On Hand) × Vehicle

Volume to administer = (Desired dose ÷ On hand dose) × Vehicle volume.

Weight-Based Dosing

Dose = Weight (kg) × Dose/kg

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

BSA (m²) = √((height cm × weight kg) / 3600)

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 mg1 gr = 60 mg
mL to L÷ 1000
tsp to mL1 tsp = 5 mL
tbsp to mL1 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.

References

Public safety references for medication calculations and dosing verification.

Medical Disclaimer

The content on this page is provided for educational and quick-reference purposes only and does not constitute medical advice. It is not a substitute for professional clinical judgment, your facility's protocols, or the guidance of a qualified licensed healthcare provider. Always verify dosages, reference ranges, and protocols against current evidence-based clinical guidelines and institutional policy before providing care. rnref.com makes no warranty regarding the accuracy or completeness of this information and is not liable for clinical decisions made based on it. See our Terms of Service, Privacy Policy, and Accessibility Statement.