This Free Water Deficit Calculator estimates FWD based on patient data, measured and ideal plasma sodium.
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)