Free Water Deficit Calculator
Estimate the free water deficit in hypernatraemia — how many litres of electrolyte-free water the body is short of, and therefore roughly how much needs replacing to bring the sodium back towards normal. Enter the weight, the measured sodium and a total-body-water fraction for the age and sex, and optionally a target sodium (140 mmol/L by default).
About the free water deficit
The free water deficit estimates the volume of electrolyte-free water needed to correct hypernatraemia, guiding replacement alongside the treatment of ongoing losses and the underlying cause. It is a standard bedside estimate derived from total body water and the ratio of the measured to the desired sodium.
Frequently asked questions
What is the free water deficit formula?
Free water deficit (L) = total body water × (measured Na ÷ target Na − 1). Total body water (TBW) is body weight in kg multiplied by a fraction: about 0.6 for children and adult men, 0.5 for adult women and elderly men, and 0.45 for elderly women. The target sodium is usually 140 mmol/L.
How do I use the result?
The free water deficit is the volume of electrolyte-free water needed to return the sodium to the target, before accounting for ongoing losses. Ongoing insensible and urinary free-water losses (and any maintenance needs) must be added on top. The route and rate of replacement depend on the clinical situation.
How fast should the sodium be corrected?
In chronic hypernatraemia the sodium is usually lowered slowly — commonly no faster than about 10 mmol/L in 24 hours — to avoid cerebral oedema. This calculator gives the total deficit, not a rate; the correction is spread over the appropriate time and the sodium is rechecked frequently. Acute hypernatraemia may be corrected faster. Always follow local guidance.
Which total body water fraction should I choose?
TBW falls with age and is lower in women because of differences in body composition. Typical fractions are 0.6 (child, adult male), 0.5 (adult female, elderly male) and 0.45 (elderly female). These are population estimates; the true value varies with muscle and fat mass, so treat the deficit as an approximation and titrate to repeated measurements.
When does this calculation apply?
The free water deficit calculation applies to hypernatraemia — a measured sodium above the target. If the sodium is at or below the target there is no free water deficit to replace, and a different problem (or overcorrection) should be considered. This calculator is an educational reference, not medical advice.
This is a reference tool, not a diagnosis or medical advice. Results depend on the laboratory, assay and clinical context, and reference intervals vary between labs and by age and sex — always read your result against the range printed on your own report and discuss it with a qualified healthcare professional.
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