How it's calculated
Total body water is estimated as 60% of body weight in men and 50% in women. Free water deficit (L) = Total body water × ((current sodium ÷ 140) − 1), estimating the volume of free water needed to correct sodium to a normal value of 140 mmol/L.
Total body water fraction used
| Group | Fraction of body weight as water |
|---|---|
| Adult male | 0.6 |
| Adult female | 0.5 |
Clinical use
- This calculates the deficit relative to a target sodium of 140 mmol/L — it estimates the volume of free water (e.g. as 5% dextrose) needed, not a rate of administration.
- Correct chronic hypernatraemia slowly — generally no faster than 8-10 mmol/L per 24 hours — since overly rapid correction risks cerebral oedema, particularly in chronic (rather than acute) hypernatraemia.
- This estimate doesn't account for ongoing free water losses (e.g. from fever, diarrhoea, diabetes insipidus) — these need to be added on top of the calculated deficit and reassessed as treatment progresses.
- Elderly patients and those with reduced lean body mass tend to have a lower total body water fraction than these standard estimates — interpret the result as a starting point, not an exact figure, and recheck sodium regularly during correction.
Frequently asked questions
What is the free water deficit calculator used for?
It estimates how much free water is needed to correct high serum sodium (hypernatraemia) back toward a normal level, based on body weight, sex, and current sodium.
Why does total body water differ between men and women?
Men generally have a higher proportion of lean muscle mass (which holds more water) and less fat mass (which holds less) than women of the same weight, so total body water is estimated as a higher fraction of body weight in men (60%) than women (50%).
How fast should hypernatraemia be corrected?
Chronic hypernatraemia should generally be corrected slowly — no faster than about 8-10 mmol/L per 24 hours — since correcting too quickly can cause cerebral oedema, particularly when the hypernatraemia has developed gradually.
Does this calculation account for ongoing fluid losses?
No — it only estimates the existing deficit relative to a target sodium of 140 mmol/L. Ongoing losses (from fever, diarrhoea, diabetes insipidus, etc.) need to be estimated and added separately, with sodium rechecked regularly during correction.
References
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342:1493-1499.
- Sterns RH. Disorders of plasma sodium — causes, consequences, and correction. N Engl J Med. 2015;372:55-65.
- Spasovski G, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170:G1-47.