How this calculator works
The deficit equation holds body water fixed. Hyponatraemia can result from excess water rather than an absolute sodium shortage, so this amount is not automatically a replacement requirement. The target is user supplied and does not define a safe treatment endpoint. The calculation excludes continuing water and electrolyte losses. The result is an estimate for information only and is not medical advice.
Formula
Sodium deficit (mmol) = TBW (L) × (desired Na - current Na in mmol/L). TBW = weight in kg × selected body-water fraction.
Worked example
For 70 kg at a 0.60 fraction, TBW is 42 L. A difference from 125 to 130 mmol/L gives 210 mmol. For 60 kg at a 0.50 fraction and the same difference, the result is 150 mmol.
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Frequently asked questions
Is the result in mmol or mmol/L?
It is an amount in mmol: litres cancel the litre denominator in the concentration difference. For sodium, mmol and mEq are numerically equal.
Can I infer a fluid volume from this alone?
No. An infused solution changes body water as well as sodium content, and clinical monitoring is required.
Can I use an individual body-water fraction?
Yes, with the custom option. Default fractions are approximate adult values and do not replace a measured or clinically assessed volume.