Free Water Deficit Calculator

Hypernatremia Overview

Hypernatremia (serum Na > 145 mmol/L) almost always reflects a free-water deficit rather than sodium excess. Causes include impaired thirst (elderly, sedated), insufficient water intake, increased water losses (diabetes insipidus, osmotic diuresis, GI losses, burns, fever), or rarely sodium overload (hypertonic saline, sea-water ingestion).

Total Body Water (TBW)

• Adult male / child: 0.6 × weight (kg)
• Adult female / elderly male: 0.5 × weight
• Elderly female: 0.45 × weight

Safe Correction Rate

Lower serum sodium by no more than 10 mmol/L in 24 hours (some guidelines say 12 mmol/L) to avoid cerebral edema, especially in chronic hypernatremia (>48 h duration). Acute hypernatremia (<24 h) may be corrected faster.

Ongoing Losses

Add ongoing free water losses (insensible ~30 mL/kg/day, urine output, GI losses) to the calculated deficit when prescribing replacement fluids — typically D5W or hypotonic saline (0.45% NaCl).