How this calculator works
The closed-system equation estimates a net water gain while solute content stays constant. It excludes ongoing losses and maintenance requirements. The cited review uses 140 mmol/L as the reference; entering another target is an algebraic generalization of that same mass-balance equation. The target does not specify how quickly to correct sodium. The result is an estimate for information only and is not medical advice.
Formula
Water deficit (L) = TBW × (current Na / desired Na - 1). TBW = weight (kg) × body-water fraction. Current Na = desired Na × (1 + deficit/TBW).
Worked example
At 70 kg and fraction 0.60, TBW is 42 L. Current Na 154 and desired Na 140 give 42 × 0.1 = 4.2 L. At 60 kg and fraction 0.50, Na 168 to 140 gives 6 L.
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Frequently asked questions
Is this a daily drinking target?
No. It is a modelled deficit, and route, timing and ongoing losses require individual clinical assessment.
Why can a body-water fraction be changed?
Age, body composition and water depletion alter body water. The approximate adult presets may not fit an individual.
What if current and desired sodium are equal?
The deficit is zero. That zero difference cannot identify body weight in an inverse calculation.