Enter the patient group, weight and serum sodium to calculate the free water deficit in liters needed to bring sodium down to the target in hypernatremia.
Last updated: November 8, 2026
The water deficit formula in hypernatremia
When blood sodium rises above normal (hypernatremia, usually over 145 mmol/L), the body is short of water relative to sodium. The free water deficit estimates how much water it takes to bring sodium back to the target: • Total body water (TBW) = fraction × weight (kg) • Free water deficit = TBW × (measured sodium ÷ target sodium − 1) Fractions: child and adult man 0.6; adult woman and older man 0.5; older woman 0.45. Example: an adult man of 70 kg with a sodium of 150 mmol/L has TBW = 0.6 × 70 = 42 L and a deficit of 42 × (150 ÷ 140 − 1) = 3.0 L. The formula does not include ongoing losses (urine, sweat, diarrhea). Lowering sodium too fast risks cerebral edema; a limit of about 10–12 mmol/L per day is commonly advised. This tool is for healthcare professionals' information and does not replace clinical judgment.
Common questions about the free water deficit
In hypernatremia, it is the estimated amount of salt-free water the body needs to bring blood sodium back to normal.
Other tools you may find useful
Current body weight in kg.
Measured value (mmol/L = mEq/L).
Usually 140 mmol/L.