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Tonicity reflects the effect of non-penetrating solutes on water movement across cell membranes; solutes that cross membranes may not maintain the same osmotic gradient. An isotonic solution is referenced to 0.9% sodium chloride, with a freezing point depression of –0.52 °C. The lesson introduces two calculation approaches: the cryoscopic method, which uses freezing point depression, and the sodium chloride-equivalent method. It also covers White–Vincent and Sprowls approaches, which use drug E values to guide the volume or amount of diluent needed. Electrolyte behavior, osmotic coefficients, route-specific tonicity adjusters, and the consequences of hypertonic or hypotonic formulations provide context for applying these calculations safely.