Concentration and Percentage Calculations for PTCB

Types of Percentage Concentrations

The PTCB exam tests three types of percentage concentrations. Weight/volume (w/v) expresses grams of solute per 100 mL of solution and is the most common in pharmacy. Volume/volume (v/v) expresses mL of solute per 100 mL of solution. Weight/weight (w/w) expresses grams of solute per 100 grams of mixture.

For example, a 1% w/v solution contains 1 gram of drug per 100 mL. If you need 500 mL of a 2% solution, you need 10 grams of drug (2 g/100 mL times 5).

Ratio Strength

Ratio strength expresses concentration as 1 part drug in a given number of parts solution. A 1:1000 solution means 1 gram in 1000 mL. To convert between percentage and ratio strength, remember that 1% equals 1:100.

Epinephrine 1:1000 is a commonly tested example, meaning 1 mg per mL (or 1 g per 1000 mL).

Study and Practice

Concentration calculations appear on every PTCB exam. Build your skills with our flashcard drills and test yourself under pressure with mini practice exams.