Introduction
In hypernatremia, the body has too little water relative to sodium. The free water deficit estimates how much water needs to be replaced to bring sodium back to a target level.
How to use the Free Water Deficit (Hypernatremia) Calculator
- Enter the patient's weight in kg.
- Enter the current measured sodium in mEq/L.
- Select sex, which sets the total-body-water fraction used in the formula.
- Enter the desired target sodium.
- Click Calculate to see the estimated deficit in litres.
Formula and Calculation Method
Free water deficit (L) = Total body water fraction × Weight (kg) × [(Current Na⁺ ÷ Target Na⁺) − 1]
Visual breakdown
REFERENCE
Correct sodium no faster than 8–10 mEq/L per 24 hours
Why use this calculator
This formula is easy to transpose incorrectly by hand. The calculator computes it directly and shows how the deficit would change if the current sodium reading were slightly different, useful for sensitivity-checking a lab value.
FAQs
Why does sex matter?
Total body water as a fraction of weight differs between sexes (roughly 0.6 for men, 0.5 for women), which scales the deficit.
How fast should the deficit be replaced?
Most guidelines recommend correcting no faster than 8–10 mEq/L in 24 hours to avoid cerebral oedema.
Does this include ongoing losses?
No — this is the free water deficit only; ongoing insensible and urinary losses must be added separately.