Free Water Deficit Calculator
Estimate free water deficit in hypernatremia using total body water fraction by age/sex.
Understanding this tool
Why nurses use it
Free water deficit estimates how much hypotonic fluid is needed to correct hypernatraemia — planning must respect correction rate limits to avoid cerebral oedema.
When to use it
Use in hypernatraemia management alongside serial sodium monitoring and prescriber orders.
Calculate
How to use this calculator
- Enter weight, sex/age TBW fraction, and current sodium.
- Review deficit volume.
- Correct slowly per protocol — typically no faster than 10–12 mEq/L per 24h unless specialist guidance.
How the calculation works
FWD = TBW × [(Na − 140) ÷ 140]. TBW fraction varies by sex and age (e.g. 0.6 adult male, 0.5 adult female).
Formula used by this tool — verify units before applying at the bedside.
Worked example
Limitations & clinical notes
- Ongoing free water losses continue during correction.
- Pediatric TBW fractions differ.
- Rapid correction risks osmotic demyelination/cerebral edema — follow local hypernatremia protocol.
References
Key sources: Adrogué HJ, Madias NE — Hypernatremia (N Engl J Med 2000).
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Disclaimer: This calculator is for educational and bedside double-check purposes only. It does not replace institutional protocols, prescriber orders, pharmacist verification, or clinical judgment. Always follow your local policies — especially for high-risk medications, infusions, pediatrics, and critical care. If the patient is acutely unwell, escalate immediately rather than relying on any calculator output.