Pharmacy Dosage Calculations: PTCB Basics

Ratio and Proportion Method

Dosage calculations are a guaranteed part of the PTCB exam. The ratio-proportion method is the most straightforward approach. Set up the known ratio on one side and the unknown on the other, then cross-multiply and solve.

For example, if a medication comes as 250 mg per 5 mL and the dose is 500 mg, you set up: 250 mg / 5 mL = 500 mg / x mL. Cross-multiplying gives x = 10 mL.

Dimensional Analysis

Dimensional analysis uses conversion factors to cancel units until you reach the desired unit. This method is especially useful for multi-step problems. Start with what you know, multiply by conversion factors, and cancel units that appear in both numerator and denominator.

Always double-check that your final answer has the correct units and that the result makes clinical sense. An answer that seems unreasonably large or small should be recalculated.

Practice Makes Perfect

Calculation questions require practice. Work through problems in our PTCB flashcards and take timed mini exams to build speed and confidence with dosage math.