Free Water Deficit Calculator

About this calculator

This calculator estimates the free water deficit in hypernatremia using Deficit = TBW × (Na⁺current/Na⁺target − 1), where TBW (total body water) is approximated as a fraction of body weight (roughly 0.6 for men, 0.5 for women, adjusted lower for the elderly), and Na⁺ values are the patient's current and target serum sodium in mEq/L. The result estimates how much pure water is missing relative to sodium, guiding how much free water needs to be replaced.

Emergency medicine and internal medicine physicians, ICU and nephrology nurses, and medical students on clinical rotations use this to guide correction of hypernatremia — a calculation that must be paired with a safe correction rate (typically no faster than 10–12 mEq/L per 24 hours) to avoid cerebral edema from correcting too quickly. Enter the patient's weight, sex, current sodium, and target sodium to get the estimated deficit.