Pharmacy Calculations
Pharmacy calculations are the mathematical operations pharmacy technicians must perform to accurately process prescriptions, including dosage conversions, day supply calculations, concentration and dilution problems, and percentage calculations.
Exam Tip
Master day supply = quantity / daily dose, C1V1 = C2V2 for dilutions, and metric conversions (1 tsp = 5 mL, 1 kg = 2.2 lb). These formulas appear heavily on the ExCPT.
What Are Pharmacy Calculations?
Pharmacy math is a core competency for pharmacy technicians. The ExCPT tests calculation skills heavily within the Dispensing Process domain (55% of the exam). Mastering these formulas and conversions is essential for safe and accurate medication dispensing.
Essential Metric Conversions
| Conversion | Equivalent |
|---|---|
| 1 kg | 2.2 lb |
| 1 oz | 30 mL (28.35 g) |
| 1 tsp | 5 mL |
| 1 tbsp | 15 mL (3 tsp) |
| 1 cup | 240 mL (8 oz) |
| 1 pint | 473 mL (16 oz) |
| 1 gallon | 3,785 mL (128 oz) |
| 1 grain | 65 mg |
| 1 L | 1,000 mL |
| 1 g | 1,000 mg |
| 1 mg | 1,000 mcg |
Day Supply Calculation
Day Supply = Total Quantity Dispensed / Daily Dose
Example: Amoxicillin 500 mg #30, Sig: 1 cap TID
- Daily dose = 3 capsules/day
- Day supply = 30 / 3 = 10 days
Concentration and Dilution (C1V1 = C2V2)
C1 x V1 = C2 x V2
Example: Dilute 70% alcohol to make 500 mL of 30% alcohol
- 70% x V1 = 30% x 500 mL
- V1 = 214.3 mL of 70% alcohol + QS to 500 mL with water
Percentage Calculations
| Type | Formula | Example |
|---|---|---|
| % w/v | grams per 100 mL | 0.9% NS = 0.9 g NaCl per 100 mL |
| % w/w | grams per 100 g | 2% hydrocortisone = 2 g per 100 g |
| % v/v | mL per 100 mL | 70% alcohol = 70 mL per 100 mL |
Ratio and Proportion
Known ratio = Unknown ratio
mg/mL = mg/mL
Example: A solution is 250 mg/5 mL. How many mL for a 400 mg dose?
- 250 mg / 5 mL = 400 mg / X mL
- X = 8 mL
Alligation Method
Used to mix two different concentrations to get a desired concentration:
Higher % ---- Desired - Lower = Parts of higher
X
Lower % ---- Higher - Desired = Parts of lower
Exam Alert
Pharmacy calculations are heavily tested on the ExCPT. Practice day supply, concentration/dilution (C1V1 = C2V2), metric conversions, and percentage calculations. An on-screen calculator is provided, but you must know which formulas to apply.
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Related Terms
Sig Codes (Prescription Abbreviations)
Sig codes are standardized abbreviations derived primarily from Latin used on prescriptions to communicate directions for medication use, including frequency, route, timing, and special instructions.
Compounding Pharmacy
A compounding pharmacy prepares customized medications by mixing, combining, or altering ingredients to create preparations not commercially available, following USP 795 (non-sterile) and USP 797 (sterile) standards.
NDC Number (National Drug Code)
The NDC (National Drug Code) is a unique 10-digit, 3-segment numeric identifier assigned to each medication product in the United States, identifying the labeler, product, and package size.
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