Free Water Deficit Calculator

This Free Water Deficit Calculator estimates FWD based on gender, age, measured and ideal plasma sodium.

Disclaimer: Always seek a doctor’s advice in addition to using this app and before making any medical decisions. This app should NOT be considered as a substitute for professional medical service, NOR as a substitute for clinical judgement.

Variables and formula
The free water deficit estimation offers information on the volume status of the patient based on age, gender, weight and plasma sodium.
Please note that the estimation provided by the calculator may not be correct in the case of patients who suffered recent significant weight gain or loss (especially from fluid sources).
FWD is often used in the management of hypernatremia when deciding fluid replenishment, in less severe cases just by oral intake, in more severe cases via IV.
The amount of free water required to balance the deficit is calculated via:
Free water deficit = TBW x (Measured Na / Ideal Na – 1)
Where:
■ Ideal Na+ can be considered 140 mEq/L;
■ TBW is total body water = correction factor x weight in kg;
The TBW correction factors are:
Patient Factor Patient Factor
Young female 0.5 Young male 0.6
Adult female 0.5 Adult male 0.6
Elderly female 0.45 Elderly male 0.5
This means that the free water deficit formulas for adults are:
■ Female FWD = 0.5 x Weight in kg x (Measured Na / Ideal Na – 1)
■ Male FWD = 0.6 x Weight in kg x (Measured Na / Ideal Na – 1)