Free Water Deficit Calculator
Free Water Deficit Calculator
Calculate the free water deficit in hypernatraemia from body weight, measured sodium and total body water fraction.
Free Water Deficit
Water deficitWeight 70 kg, sodium 158 mmol/L, adult male total body water fraction 0.6
Formula
- TBW
- total body water in litres — weight in kg × the fraction for age and sex
- 140
- the assumed normal serum sodium the deficit is calculated against
- result
- litres of electrolyte-free water needed to return sodium to 140 mmol/L
Worked example
Weight 70 kg, sodium 158 mmol/L, adult male total body water fraction 0.6
TBW = 70 × 0.6 = 42 L
158 ÷ 140 − 1 = 0.1286
42 × 0.1286 = 5.40 L
Safe correction rates in hypernatraemia
| Duration | Maximum correction |
|---|---|
| Acute (< 48 h), known onset | Faster correction acceptable; still monitor closely |
| Chronic or duration unknown | 10 – 12 mmol/L per 24 hours, no faster |
What the deficit does, and does not, cover
The free water deficit is the volume of electrolyte-free water needed to return serum sodium to 140 mmol/L, calculated from total body water and how far measured sodium sits above normal. Total body water is estimated as weight multiplied by a fraction that falls with age, in women, and with reduced lean mass — a child, by contrast, carries a higher fraction than an adult.
The correction rate matters as much as the volume. In chronic hypernatraemia — present for more than 48 hours, or of unknown duration, which should be treated as chronic by default — sodium should be corrected no faster than 10–12 mmol/L in 24 hours. Brain cells adapt to a hyperosmolar state by accumulating intracellular osmoles to limit water loss; correcting too quickly lets water re-enter faster than those osmoles can be cleared, causing cerebral oedema. Faster correction is acceptable only when the hypernatraemia is known to be acute and of short duration, where that adaptation has not yet occurred.
The formula gives the water deficit alone. It takes no account of ongoing insensible or urinary losses, which must be added separately and reassessed as treatment progresses, and it says nothing about circulating volume. A hypotensive or volume-depleted patient needs isotonic fluid for resuscitation first — replacing the free water deficit with hypotonic fluid before restoring perfusion is a common and avoidable error.
Frequently asked questions
What does the free water deficit calculator estimate?
The volume of electrolyte-free water needed to bring serum sodium back to 140 mmol/L, based on total body water and how far above normal the measured sodium sits.
How fast can hypernatraemia safely be corrected?
No faster than 10–12 mmol/L in 24 hours if the hypernatraemia is chronic or of unknown duration, to avoid cerebral oedema. Faster correction is acceptable only when the onset is known to be acute and recent.
Does this formula account for ongoing fluid losses?
No. It calculates the existing deficit only. Continuing insensible and urinary losses must be estimated and added separately, and reassessed as treatment continues.
What if the patient is also hypotensive?
Restore circulating volume with isotonic fluid first. The free water deficit addresses the sodium concentration, not perfusion, and treating volume depletion with hypotonic fluid delays necessary resuscitation.
Related calculators
References
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493–1499.
- Sterns RH. Disorders of plasma sodium — causes, consequences, and correction. N Engl J Med. 2015;372(1):55–65.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.