Free Water Deficit Calculator (Hypernatremia)
Enter the serum sodium, weight and patient group. For clinicians and students.
Your numbers
Your results
Free Water Deficit Calculator (Hypernatremia) by TheCalcTools
How the Free Water Deficit Calculator (Hypernatremia) works
Free water deficit = total body water × (current sodium ÷ target sodium − 1). Total body water is estimated as a fraction of weight: 0.6 for adult men and children, 0.5 for women and older men, 0.45 for older women.
Correction should be gradual, as rapid falls in chronic hypernatremia risk cerebral oedema; ongoing losses must be added. This is a teaching estimate, not a treatment plan.
The formula
Worked example
Using the default values shown in the calculator:
| Serum sodium (mmol/L) | 155 |
|---|---|
| Weight (kg) | 70 |
| Patient group (total body water fraction) | Adult man (0.6) |
| Target sodium (mmol/L) | 140 |
- Sodium 155 in a 70 kg man: deficit 4.50 L.
Result:
| Free water deficit (L) | 4.50 |
|---|---|
| Total body water (L) | 42.0 |
| Maximum safe sodium fall in 24 h (common limit) | About 10 mmol/L per 24 hours in chronic hypernatremia |
Frequently asked questions
Why correct slowly?
Brain cells adapt to high sodium; lowering it too fast draws water into them.
Sources and further reading
Related calculators
More in Health & Fitness · All calculators
This calculator gives general estimates for information and education. It is not medical advice and does not diagnose anything. Talk to a qualified health professional about your own situation. See our disclaimer.