% = (solute / solution) × 100
There are three types of percent concentration: (1) w/v% = grams of solute per 100 mL of solution — most common in medicine and biology (e.g., 0.9% NaCl = 0.9 g in 100 mL). (2) w/w% = grams of solute per 100 g of solution — used in food, pharmacy, and industrial chemistry (e.g., 3% hydrogen peroxide). (3) v/v% = mL of solute per 100 mL of solution — used for liquid-liquid mixtures (e.g., 70% ethanol). Conversions between types require density: w/v% = w/w% × density. In pharmacy, percent solutions are critical for drug formulation — 1% w/v = 10 mg/mL = 10,000 ppm. Always specify the type to avoid ambiguity in preparation and dosing.