Drug Dosage Calculations for Nurses
Drug calculation is a core NMCN skill. Nurses must accurately compute doses, tablets, and volumes using standard formulas to administer medication safely.
Practice Drug Calculations with PassMate AI →Key Points to Know
What you must know about Drug Calculations
Basic dosage formula: Dose = (Desired dose ÷ Stock strength) × Stock volume.
Always convert units to the same measure before calculating (e.g., g to mg, mg to mcg).
1 g = 1000 mg; 1 mg = 1000 mcg; 1 L = 1000 mL.
For tablets: Number of tablets = Desired dose ÷ Strength per tablet.
Always double-check high-alert drug calculations (insulin, heparin, opioids).
When in doubt, verify with a second nurse or the pharmacist before administering.
NMCN Exam Tips
How this topic appears in the NMCN exam
Convert all units to match BEFORE applying the formula — a common trap.
Show the "desired over have times volume" working to avoid errors.
Watch for answers that are off by a factor of 10 due to unit errors.
Paediatric doses must be checked against maximum safe dose.
Practice Question
Test yourself
A doctor prescribes 250 mg of a drug. The stock available is 500 mg in 2 mL. How many mL should the nurse administer?
Explanation
Using Dose = (Desired ÷ Have) × Volume = (250 ÷ 500) × 2 mL = 0.5 × 2 = 1 mL. The nurse should administer 1 mL.
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