Percentage & Ratio Strength Practice Problems
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Percentage strength and ratio strength are two ways of expressing how much drug is in a preparation. Percent weight-in-volume (% w/v) is grams of drug per 100 mL — the most common form — and ratio strength (1:X) conventionally means 1 gram of drug in X mL of preparation. Converting fluidly between percent, ratio strength, and mg/mL is the foundation for nearly every other pharmacy calculation.
Formula% w/v = grams of drug per 100 mL • 1:X ratio = 1 g in X mL • mg/mL ÷ 10 = % w/v
Worked example
Express 1:2,500 as a percentage. 1:2,500 means 1 g in 2,500 mL, so (1 ÷ 2,500) × 100 = 0.04% w/v.
💡 Anchors to memorize: 1:1,000 = 0.1% (epinephrine), 1:10,000 = 0.01%.