Dosage Calculations for Nursing Students: Formulas & Practice

Essential Dosage Calculation Formulas

Formula Method (Desired Over Have)

Dose = (Desired / Have) × Vehicle

Example: Order: Amoxicillin 500 mg PO. Available: 250 mg/5 mL

• Dose = (500 / 250) × 5 mL = 10 mL

Dimensional Analysis

Set up conversion factors to cancel units systematically.

Example: Order: Morphine 4 mg IV. Available: 10 mg/mL

• 4 mg × (1 mL / 10 mg) = 0.4 mL

IV Flow Rate Calculations

mL/hr (Pump Rate)

mL/hr = Total Volume (mL) / Time (hours)

Example: Infuse 1000 mL NS over 8 hours

• 1000 / 8 = 125 mL/hr

gtts/min (Drip Rate)

gtts/min = (Volume × Drop Factor) / (Time in minutes)

Drop factors: Macro = 10, 15, or 20 gtts/mL; Micro = 60 gtts/mL

Example: 1000 mL over 8 hours, drop factor 15

• (1000 × 15) / (8 × 60) = 15000 / 480 = 31 gtts/min

Weight-Based Calculations

mg/kg Dosing

Dose = Weight (kg) × Dose (mg/kg)

Example: Patient weighs 70 kg. Order: Vancomycin 15 mg/kg

• 70 × 15 = 1050 mg

Pediatric Safe Dose Range

Always calculate the safe dose range and compare to the ordered dose.

Common 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

Tips for Accuracy

1. Always double-check using a second method
2. Label all units: this prevents errors
3. Round appropriately: typically to nearest tenth for mL
4. Know your "reasonable" ranges: if the answer seems too high/low, recalculate
5. Convert first, calculate second: get everything in the same units

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