Pharmacy Math Conversion Factors for the PTCB Exam

Master These Conversions for the PTCB Exam

Pharmacy calculations on the PTCB certification exam require you to convert between different measurement systems quickly and accurately. This comprehensive guide covers every conversion factor you need to memorize for exam day.

Metric System Conversions

The metric system is the standard in pharmacy. Key conversions:

Weight: 1 kg = 1,000 g; 1 g = 1,000 mg; 1 mg = 1,000 mcg
Volume: 1 L = 1,000 mL
Length: 1 m = 100 cm; 1 cm = 10 mm

Memory tip: In the metric system, each step up or down is a factor of 1,000 for weight and volume. Moving to a smaller unit, multiply by 1,000. Moving to a larger unit, divide by 1,000.

Household to Metric Conversions

Patients often measure medications using household units. Essential conversions:

1 teaspoon (tsp) = 5 mL
1 tablespoon (tbsp) = 15 mL = 3 tsp
1 fluid ounce (fl oz) = 30 mL = 2 tbsp
1 cup = 240 mL = 8 fl oz
1 pint = 480 mL = 16 fl oz
1 quart = 960 mL = 32 fl oz
1 gallon = 3,840 mL = 128 fl oz

Weight Conversions

1 kg = 2.2 lbs (most critical conversion for weight-based dosing)
1 lb = 454 g
1 oz = 28.35 g (approximately 30 g)
1 grain (gr) = 65 mg (approximately 60 mg in some references)

Temperature Conversions

Temperature conversions are important for medication storage:

Celsius to Fahrenheit: F = (C x 9/5) + 32
Fahrenheit to Celsius: C = (F - 32) x 5/9
Key reference points: Body temperature 37 C = 98.6 F; Freezing 0 C = 32 F; Room temperature approximately 20-25 C = 68-77 F; Refrigerator 2-8 C = 36-46 F

Concentration Expressions

Understanding how concentrations are expressed is essential for calculations:

% w/v (weight-in-volume): Grams per 100 mL (e.g., 0.9% NaCl = 0.9 g per 100 mL)
% w/w (weight-in-weight): Grams per 100 g (used for creams, ointments)
% v/v (volume-in-volume): mL per 100 mL (used for liquid-in-liquid solutions)
Ratio strength: 1:1000 = 1 g per 1,000 mL (e.g., epinephrine 1:1000)
mg/mL: Direct expression of concentration

Roman Numerals

Some prescriptions still use Roman numerals:

I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1000
ss = 1/2 (e.g., "iss" = 1.5)

Quick Reference Formulas

Day supply: Total quantity / Daily dose = Days
Dilution: C1V1 = C2V2
Alligation: Used to mix two concentrations to achieve a desired concentration
Flow rate: Volume (mL) / Time (hours) = mL/hr
Drip rate: (Volume x Drop factor) / (Time in minutes) = drops/min

Practice Makes Perfect

Memorize these conversions and practice applying them in calculation problems. The more you practice, the faster and more accurate you will become. Work through calculation problems daily at PTCBprep and track your progress at your dashboard.