Free Water Deficit (Hypernatremia, Teaching)
Water deficit (L) ≈ TBW × ([Na]/140 − 1) when serum Na > 140; TBW ≈ 0.6×kg (men) or 0.5×kg (women).
Calculator
How it works
Classic teaching estimates how much water would be needed to dilute extracellular sodium toward a reference concentration if TBW were a single well-mixed compartment (it is not).
TBW (L) ≈ 0.6 × weight (men) or 0.5 × weight (women) Free water deficit (L) ≈ TBW × ( [Na]/140 − 1 )
Limits
Does not replace pediatric formulas, hypertonic saline strategies, or diabetes insipidus workups.
Educational tool only—not for diagnosis or treatment decisions.