Your result shows the estimated volume of free water (in liters) that the patient has lost, leading to the current hypernatremic state. This deficit represents the amount of electrolyte-free water that would need to be replaced to bring the serum sodium back toward a normal target of 140 mEq/L. For example, a calculated deficit of 4 liters means the patient has lost approximately 4 liters of free water relative to their current total body water.

This value is an estimate and a starting point for fluid replacement planning — it does not account for ongoing losses (insensible losses, urine output, ongoing osmotic diuresis) that continue during the correction period. The total fluid requirement will typically exceed the calculated deficit because these ongoing losses must be replaced simultaneously. Frequent monitoring of serum sodium (every 4–6 hours during active correction) is essential to ensure safe and adequate replacement.

Use the free water deficit calculator when managing a patient with hypernatremia (serum sodium > 145 mEq/L) who requires free water replacement. It is most commonly applied in the ICU, inpatient wards, and emergency department when planning fluid replacement for patients with hypernatremia due to dehydration, diabetes insipidus, osmotic diuresis, or inadequate free water intake (common in elderly or critically ill patients who cannot drink independently).

The calculation is a standard component of the nephrology and critical care approach to hypernatremia management. It helps determine how much free water to administer over a defined correction period, which is then used to set an hourly infusion rate for hypotonic fluids (D5W or 0.45% saline) or to guide enteral free water administration via feeding tube.

The free water deficit formula assumes a static state and does not account for ongoing water losses. Patients with active diuresis, fever, tachypnea, or gastrointestinal losses will continue losing free water during the correction period, and the actual replacement volume needed may be significantly higher than the calculated deficit. A comprehensive fluid plan must include both the deficit replacement and an estimate of ongoing losses.

The total body water (TBW) estimation used in the formula (60% for males, 50% for females) is an approximation that may be inaccurate in elderly patients (who have lower TBW fraction), obese patients (adipose tissue contains less water), or edematous patients. The rate of correction is critically important — for chronic hypernatremia (developing over more than 48 hours), sodium should not be lowered by more than 10–12 mEq/L per 24 hours to avoid cerebral edema. For acute hypernatremia (< 48 hours duration), faster correction is safer. The formula provides a volume target but does not itself dictate the rate, which requires clinical judgment based on the chronicity of the hypernatremia.