Pharmaceutical Calculations and Measurement
Core concepts
Master the calculation types that appear in daily practice: dose and quantity calculations, concentration and dilution, percentage strength, ratio strength, alligation, pediatric weight-based dosing, infusion rates, and unit conversions between metric and household measures. Understand significant figures and rounding rules, and know that final answers must be checked against a sensible range. Be comfortable with dimensional analysis (unit cancellation) as a single reliable method for most problems.
Traps in questions
Common traps include mixing units (mg vs. mcg, mL vs. L), forgetting to convert body weight from pounds to kilograms, and rounding too early in a multi-step problem. Questions may give extra information that is not needed, or ask for the amount per dose when the label states the amount per day. Percentage strength is a frequent stumbling block: a percentage of a solid-in-liquid preparation is weight/volume, while percentage of a liquid-in-liquid preparation is volume/volume. Always check whether the question asks for a rate (per hour) or a total.
Rule of thumb
Set up the problem so units cancel, convert all values to a single system before calculating, and do a final sanity check: does the dose look reasonable for the patient's size and the drug's usual range?
Worked example
A patient weighs 44 lb and is prescribed 5 mg/kg/day in two divided doses. Convert: 44 lb ÷ 2.2 = 20 kg. Daily dose = 5 × 20 = 100 mg/day. Per dose = 100 ÷ 2 = 50 mg. If the stock is 25 mg/5 mL, each dose is 50 ÷ 25 × 5 = 10 mL.