Pharmacy Calculations
The math that keeps patients safe.
Module overview
Calculations are roughly 21% of the PTCE and the area where most candidates lose points. The math itself is middle-school level — the danger is unit setup. Always write units, always cancel them, always sanity-check the magnitude.
What you'll learn
- 01Conversions and ratios
- 02Dosage calculations
- 03IV flow rates
- 04Days' supply calculations
Lessons
Conversions you must memorize
Pharmacy conversions are non-negotiable memorization. Drill them until they're automatic.
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Dosage calculations
Use dimensional analysis: write the desired unit on the left, multiply by ratios that cancel until you arrive at it.
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IV flow rates and drip rates
Flow rate (mL/hr) = total volume ÷ total time. Drip rate (gtt/min) = (volume × drop factor) ÷ time in minutes.
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Concentrations, dilutions, and alligation
Percent strength, ratio strength, dilution, and alligation are tested every exam.
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Days' supply
Days' supply = total quantity ÷ amount used per day. Wrong days' supply causes more insurance rejections than any other field.
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Key terms
- Dimensional analysis
- Calculation method that cancels units across multiplied ratios to reach the desired unit.
- Alligation
- Technique to determine the proportion of two strengths needed to make a target strength.
- Drop factor
- The number of drops per mL delivered by a specific IV tubing set.
- C₁V₁ = C₂V₂
- Dilution equation relating original and final concentration and volume.
Study tool
Flashcards
01 / 19
Click the card to reveal the answer.
Practice questions
- Q1Convert 154 lb to kg.
- Q2Order: 500 mg amoxicillin tid × 10 days. Stock is 250 mg/5 mL. How many mL to dispense?
- Q3How many mL of 70% alcohol and water are needed to make 500 mL of 30%?
Weekly study rhythm
- • Watch the module lecture video
- • Complete guided notes and flashcards
- • Take the end-of-lesson quiz
- • Practice pharmacy calculations daily
- • Take a mock exam every two modules