The calculator assesses the free water deficit in individuals with hypernatremia by integrating five particular inputs: gender, age group, weight, current serum sodium level, and desired serum sodium level. This metric is invaluable for determining how much free water is necessary to adjust serum sodium levels back to normal.

The calculation process unfolds through these steps:

- **Weight Conversion**: The individual’s weight can be entered in either kilograms or pounds. If pounds are used, the calculator converts this to kilograms: 
  Weight in kg = Weight in lbs / 2.20462.

- **Total Body Water (TBW) Fraction**: This fraction represents the portion of body weight that comprises water. Its determination hinges on both the individual’s gender and age group:
  - The TBW fraction is 0.6 for children, adult males, and non-elderly males.
  - For adult females, the TBW fraction adapts to 0.5.
  - In the elderly, it decreases to 0.5 for males and further to 0.45 for females.

- **Sodium Levels**: Input for serum sodium levels is accepted in mEq/L or mmol/L, given the equivalence of these units, no conversion is needed.

- **Computation of Free Water Deficit**: This quantity is calculated with the formula:
  Free Water Deficit (L) = TBW fraction × Weight (kg) × [(Current Serum Sodium / Desired Serum Sodium) - 1].

- **Outcome**: The free water deficit is calculated and then rounded to one decimal place, delivered in liters.

This calculator offers a tailored analysis essential for the customized management of hypernatremia. This consideration of individual parameters facilitates personalized fluid replacement therapy, ensuring serum sodium levels are safely normalized.

The Free Water Deficit in Hypernatremia calculator estimates the amount of free water (in liters) needed to correct hypernatremia based on the patient's sodium levels, weight, and total body water (TBW) percentage