Free Water Deficit Calculator (Hypernatraemia)
The free water deficit estimates the litres of water needed to correct hypernatraemia: deficit = total body water × (serum sodium ÷ 140 − 1), where total body water is body weight multiplied by a sex- and age-specific fraction. It is a guide to the replacement requirement, not a rate of correction.
Quick answer
The free water deficit estimates the litres of water needed to correct hypernatraemia: deficit = total body water × (serum sodium ÷ 140 − 1). Total body water is body weight times a fraction — about 0.6 in adult men, 0.5 in adult women and elderly men, and 0.45 in elderly women. It estimates the requirement, not the rate of correction.
Inputs
The deficit is calculated to a target sodium of 140 mmol/L. It estimates the water requirement, not the rate of correction.
How to use the Free Water Deficit calculator
- 1Enter weight and sodium. Enter body weight in kilograms and the current serum sodium in mmol/L.
- 2Select the body-water group. Choose the group that sets the total-body-water fraction: adult man, adult woman, elderly man, or elderly woman.
- 3Read the deficit. The tool shows the estimated free water deficit in litres to reach a sodium of 140 mmol/L. The rate and route of replacement are decided by a clinician to avoid over-rapid correction.
Medical disclaimer: This is general health information, not medical advice. It does not diagnose or treat any condition, and the results are estimates based on public reference formulas. Always consult a qualified doctor about your individual health. If you think you may have a medical emergency, contact your local emergency services immediately.
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Book a consultationFrequently asked questions
What does the free water deficit tell you?⌄
It estimates how many litres of electrolyte-free water are needed to lower the serum sodium to the target (140 mmol/L). It quantifies the requirement; it does not specify how fast to replace it — over-rapid correction of chronic hypernatraemia can cause cerebral oedema.
Why does the fraction differ by sex and age?⌄
Total body water makes up a smaller proportion of body weight in women and in older people because of differences in body composition. The standard fractions (0.6, 0.5, 0.45) reflect this so the deficit estimate is more accurate.
Does the deficit include ongoing losses?⌄
No. The formula estimates the existing water deficit only. Ongoing insensible and urinary losses must be added separately when planning replacement, which a clinician does.
Is my data stored?⌄
No. The calculator runs entirely in your browser; nothing you enter is sent to a server.
Sources & validation
This calculator reproduces the published Free Water Deficit score, validated for estimating the free water deficit in hypernatraemia.
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