Free Water Deficit Calculator

How it is calculated

The tool estimates total body water as a fixed fraction of body weight, then scales it by how far serum sodium sits above the target: TBW = 0.6 × W and deficit = TBW × (Na ÷ Natarget − 1). W is weight in kilograms, Na is the current serum sodium and Natarget the desired value, both in mmol/L (the same unit for both). Results are in litres, to two decimals. The 0.6 factor is always used; it is not adjusted for sex, age or frailty. All three inputs must be above zero. If sodium is below the target the deficit comes out negative, meaning no free water deficit. The figure is an estimate, not a prescription.

Common mistakes

• Entering weight in pounds. The 0.6 × weight step assumes kilograms, so pounds overstate the deficit by more than double.
• Mixing sodium units, such as mg/dL for one value and mmol/L for the other. Both must share a unit.
• Treating the result as the volume to give at once. It is the estimated total deficit, and the tool says nothing about the rate of correction.
• Expecting an adjustment for sex or age. The tool always uses 0.6, so elderly or female patients, who have lower body water fractions, are overestimated.
• Swapping current and target sodium. A negative result then appears for a patient who is actually hypernatraemic.