Metric Conversions
Key conversions: 1 kg = 2.2 lbs. 1 g = 1000 mg. 1 mg = 1000 mcg. 1 L = 1000 mL. 1 tsp = 5 mL. 1 tbsp = 15 mL. 1 oz = 30 mL. Temperature: °C = (°F − 32) × 5/9; °F = (°C × 9/5) + 32. Always convert to the same unit before calculating. Memorize these — they appear in every dosage calculation scenario.
Desired/Have Formula
Formula: (Desired dose ÷ Have dose) × Vehicle = Amount to give. Example: Order = amoxicillin 500 mg. On hand: 250 mg/5 mL. Calculation: (500 mg ÷ 250 mg) × 5 mL = 2 × 5 mL = 10 mL. Always label your answer with units. Always double-check with a different method. Always assess if the answer is clinically reasonable.
Dimensional Analysis
Method: set up the problem so units cancel. Start with what you have, multiply by conversion factors until you reach the desired unit. Example: Order: 250 mcg. On hand: 0.5 mg/tablet. → 250 mcg × (1 mg / 1000 mcg) × (1 tablet / 0.5 mg) = 0.5 tablet. Preferred method for multi-step problems. Write out every unit — don't skip steps.
Weight-Based Dosing
Convert patient weight to kg (lbs ÷ 2.2). Multiply dose (mg/kg) by weight (kg) = total dose in mg. Example: heparin 80 units/kg. Patient weighs 165 lbs = 75 kg. 80 × 75 = 6,000 units. Then calculate volume from concentration. Check: is the dose within normal range? Always verify with drug reference and pharmacist for high-alert medications.
Pediatric Dosing
Always weight-based (mg/kg). Verify weight in kg before every calculation — a 10-fold error in weight = 10-fold dosing error. Check maximum dose — never exceed even if weight-based calculation exceeds it. BSA method (mg/m²): used for chemotherapy. Pediatric-specific drug references (Lexi-Comp Pediatrics, Epocrates) — never extrapolate adult doses.
Important Notes
- If your calculated dose seems too high or too low — stop, recalculate, and verify. Clinical judgment is part of safe drug administration.
- Pediatric dosing errors are among the most dangerous — always double-check weight in kg and have a second nurse verify.
Clinical Tips
- Dimensional analysis is fool-proof when done correctly — teach yourself this method for all multi-step problems.
- Round to a measurable unit (e.g., 0.5 mL on a syringe) — document what was actually given, not the theoretical calculation.
Summary
Accurate dosage calculation is a foundational nursing safety skill. Mastery of metric conversions, the desired/have formula, dimensional analysis, and weight-based dosing prevents medication errors and ensures patient safety.
