Free Water Deficit Calculator

The calculation relies on evaluating the patient's existing total body water against their current and desired electrolyte concentrations. The core equation is expressed as:

FWD = K × Weight × (Sodium / Desired Sodium - 1)

Where FWD is the free water deficit in liters, Weight is measured in kilograms, Sodium is the current serum sodium concentration, Desired Sodium is the target serum sodium concentration, and K represents a constant factor reflecting total body water percentage based on the patient's age and biological sex category (typically 0.6 for adult men, 0.5 for adult women and elderly men, and 0.45 for elderly women).

Consider a 70 kg adult male patient who presents with a serum sodium level of 152 mEq/L, and the clinical objective is to safely lower this value to a safer target of 140 mEq/L.

1. Identify the body water constant for an adult man: K = 0.6.

2. Substitute the known values into the equation: FWD = 0.6 × 70 × (152 / 140 - 1).

3. Calculate the weight multiplier: 0.6 × 70 = 42.

4. Evaluate the sodium ratio: 152 / 140 ≈ 1.0857, minus 1 yields 0.0857.

5. Multiply the results: 42 × 0.0857 ≈ 3.6 liters.

Thus, the estimated free water deficit is approximately 3.6 liters, which must be administered gradually alongside ongoing fluid losses.

Never attempt to correct severe hypernatremia too quickly, as rapid shifts can cause cerebral edema and permanent neurological damage. Always factor in ongoing insensible losses and urinary output in addition to the calculated free water deficit. Re-check serum sodium levels frequently during active fluid resuscitation to ensure the rate of correction stays within safe medical guidelines, typically not exceeding 8 to 10 mEq/L over a 24-hour period.