⚕️ For clinical reference only. Not a substitute for professional judgment.
Free Water Deficit Electrolytes
Estimates free water deficit in hypernatraemia to guide safe replacement. Correct no faster than 10–12 mmol/L/day to avoid cerebral oedema.
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Enter patient details
Enter weight, sex, and current sodium to calculate free water deficit.
Patient Parameters
kg
mmol/L
mmol/L (usually 140)
Formula & Safe Rate
TBW = sex factor x weight (kg)
Free Water Deficit = TBW x ((Na / target) - 1)
Safe correction: max 10-12 mmol/L/day
Divide deficit over 48-72 hours
Add insensible losses (~1-1.5 L/day) to daily requirements
Use oral water or IV 5% dextrose
When to use
- Hypernatraemia >145 mmol/L — identify and treat underlying cause
- HHS — alongside corrected sodium for fluid planning
- Neurogenic diabetes insipidus — estimate ongoing losses
- Never correct faster than 10 mmol/L/day (risk of cerebral oedema)