Drug calculations intimidate many nursing students, but they don't have to. With the right approach and consistent practice, dosing becomes second nature. Let's break down the essential formulas.
The Foundation: Desired Over Have
This single formula solves most medication problems:
Dose = (Desired ÷ Have) × Vehicle
Example: Order: Amoxicillin 500mg. Have: Amoxicillin 250mg/5mL
(500 ÷ 250) × 5mL = 2 × 5mL = 10 mL
(500 ÷ 250) × 5mL = 2 × 5mL = 10 mL
Weight-Based Dosing
Dose = Weight (kg) × mg/kg ordered
First, convert pounds to kg: lbs ÷ 2.2 = kg
Example: Order: Vancomycin 15 mg/kg. Patient weighs 176 lbs.
176 ÷ 2.2 = 80 kg
80 × 15 = 1200 mg
176 ÷ 2.2 = 80 kg
80 × 15 = 1200 mg
IV Drip Rates
gtt/min = (Volume × Drop Factor) ÷ Time in minutes
Drop Factors: Macrodrip = 10, 15, or 20 gtt/mL | Microdrip = 60 gtt/mL
Example: Infuse 1000mL over 8 hours using 15 gtt/mL tubing.
(1000 × 15) ÷ (8 × 60) = 15000 ÷ 480 = 31 gtt/min
(1000 × 15) ÷ (8 × 60) = 15000 ÷ 480 = 31 gtt/min
mcg/kg/min Drips
mL/hr = (mcg/kg/min × weight kg × 60) ÷ concentration (mcg/mL)
Example: Dopamine 5 mcg/kg/min for 70 kg patient. Concentration: 800 mcg/mL
(5 × 70 × 60) ÷ 800 = 21000 ÷ 800 = 26.25 mL/hr
(5 × 70 × 60) ÷ 800 = 21000 ÷ 800 = 26.25 mL/hr
Practice Makes Perfect
Do at least 5 calculation problems every day. Start with simple ones and gradually add complexity. Always double-check your work - in real practice, calculation errors can be fatal.
Drug Calculation Reference Sheet
All the formulas, conversions, and examples on one printable page.
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