Free Water Deficit Calculator
Calculate the free water deficit (FWD) to estimate the amount of water needed to correct hypernatremia. This tool uses the standard formula based on body weight, current sodium, and target sodium.
Published by:
isCalculator Editorial Team
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How to use this tool?
How to use the Free Water Deficit Calculator
- Enter Weight (kg): Input the patient’s weight in kilograms. Only metric units are accepted.
- Select Gender: Choose Male or Female. This determines total body water (TBW): 60% for males, 50% for females.
- Enter Current Serum Na⁺ (mEq/L): Input the measured sodium level (must be higher than the target).
- Enter Target Serum Na⁺ (mEq/L): Input the desired sodium level (e.g., 145 mEq/L).
- Click “Calculate”: The result shows the free water deficit in liters. The formula used is: TBW × (1 − TargetNa / CurrentNa).
Note: Current Na must be greater than Target Na. If not, a warning will appear. All inputs are required.
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Frequently Asked Questions
Free water deficit is the estimated volume of water needed to correct hypernatremia (high sodium levels). It helps guide fluid replacement therapy.
The formula is: Deficit = Total Body Water (TBW) × (1 - Target Na⁺ / Current Na⁺). TBW is estimated as 0.6 × weight (kg) for males and 0.5 × weight for females.
Normal serum sodium levels range from 135 to 145 mEq/L. Hypernatremia is defined as sodium > 145 mEq/L.
