Free water deficit formulas estimate the volume of water needed to lower serum sodium from the measured value to a chosen target, assuming a fixed total body water (TBW) fraction based on sex, age and body composition. This calculator applies those equations to adults and children with hypernatraemia who are haemodynamically stable or have been stabilised, to give a starting point for planning water replacement.

The TBW fractions used here (0.6 for adult men and children, 0.5 for adult women and elderly men, 0.45 for elderly women) are consistent with standard clinical references derived from body-composition data. Adrogué and Madias’ review and subsequent nephrology literature popularised these water-deficit equations for hypernatraemia management, while also highlighting that they provide only an approximation rather than an exact prescription.

Paediatric and adult dysnatraemia guidelines emphasise that calculated deficits do not account for initial volume resuscitation, insensible losses or ongoing renal and gastrointestinal losses, and that hypernatraemia carries significant mortality, especially in infants and the critically ill. Most recommend limiting the fall in serum sodium to roughly 10–12 mmol/L per 24 hours in chronic or unknown-duration hypernatraemia, correcting large deficits over at least 48 hours, and monitoring sodium frequently during the early phase of treatment.

Use this estimate alongside clinical assessment of volume status, neurological state, urine output and ongoing fluid balance. Accuracy is reduced in extremes of body size, oedematous states and situations where hypernatraemia is driven mainly by sodium gain rather than water loss, so individualise therapy and seek senior or specialist advice when correction is complex.

Free Water Deficit in Hypernatraemia