Free-water deficit in hypernatraemia

Acid-base & electrolytes

How much water the patient is short. Replace slowly: the target fall in sodium is ≤10 mmol/L per 24 h.

What you enter

Serum sodium (mmol/L)
Weight (kg or lb)
Total body water fraction: Adult male (0.6) / Adult female (0.5) / Older male (0.5) / Older female (0.45) / Child (0.6)

FORMULADeficit (L) = TBW × weight × (Na/140 − 1)

SourceAdrogué HJ, Madias NE. Hypernatremia. NEJM 2000;342:1493-9ReviewedFormula and thresholds checked against the source on 2026-08-22. If the source updates, this line will say so.