0.6
42
L
0.071
3
L
1
days
3
L/day
0.6
42
L
0.071
3
L
1
days
3
L/day
The Free Water Deficit calculator is a critical clinical tool used in emergency medicine, internal medicine, and critical care to quantify the volume of hypotonic fluid needed to correct hypernatremia (elevated serum sodium). Hypernatremia, defined as serum sodium above 145 mEq/L, indicates a relative deficit of water to sodium in the body and requires careful correction to avoid neurological complications. This calculator estimates the free water deficit based on patient demographics and current serum sodium level.
Hypernatremia most commonly results from inadequate water intake (particularly in elderly or cognitively impaired patients), excessive water losses (diabetes insipidus, osmotic diuresis, burns, diarrhea), or rarely from sodium excess. In hospitalized patients, hypernatremia is associated with increased mortality, particularly in the elderly and critically ill. Rapid identification and appropriate correction of the free water deficit is a cornerstone of hypernatremia management.
The free water deficit formula estimates Total Body Water (TBW) using gender and age-specific correction factors (0.6 for young males, 0.5 for elderly males, 0.5 for young females, 0.45 for elderly females), then calculates the deficit as: TBW x ((serum sodium / 140) - 1). The target sodium of 140 mEq/L represents the midpoint of the normal range (135-145 mEq/L). The calculated deficit represents the minimum volume of pure water (or hypotonic fluid) needed to normalize serum sodium, assuming no ongoing losses.
The rate of correction is critically important. For acute hypernatremia (developing over less than 24 hours), rapid correction is safe and recommended. For chronic hypernatremia (developing over more than 48 hours or unknown duration), serum sodium should be lowered no faster than 10-12 mEq/L per 24 hours to prevent cerebral edema. Overly rapid correction of chronic hypernatremia can cause brain swelling because brain cells have adapted to the hyperosmolar environment by accumulating intracellular osmoles.
This calculator provides the total free water deficit and a maximum correction rate of 10 mEq/L per day as a safety guideline. In practice, the actual replacement fluid, rate, and monitoring schedule must be determined by the treating physician based on the clinical context, ongoing losses, the patient's ability to drink, and serial sodium measurements. The calculated deficit is a starting point — ongoing losses must be added, and frequent lab monitoring (every 4-6 hours initially) is essential to ensure safe correction.
TBW estimation: Male under 65: weight x 0.6, Male 65+: weight x 0.5, Female under 65: weight x 0.5, Female 65+: weight x 0.45. Free Water Deficit = TBW x ((Serum Na / 140) - 1). Target sodium is 140 mEq/L (mid-normal range). Maximum correction rate is 10 mEq/L per 24 hours for chronic hypernatremia to avoid cerebral edema.
The free water deficit shows how much pure water is needed to normalize sodium. In practice, replacement fluids (D5W, 0.45% saline) are used instead of pure water. The deficit does not account for ongoing losses (insensible, renal, GI), which must be added. Correct chronic hypernatremia slowly (max 10-12 mEq/L/day). Monitor sodium every 4-6 hours during correction. This is a clinical tool — always use under physician guidance.
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A 72-year-old male with Na 155 has TBW of 35L and a free water deficit of ~3.8L.
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A 30-year-old female with Na 148 needs about 1.7L of free water to correct.
It is the volume of pure water needed to dilute body fluids back to a normal sodium concentration. It occurs when the body loses more water than sodium, or when sodium is gained in excess of water, causing hypernatremia.
Hypernatremia is serum sodium above 145 mEq/L. Mild hypernatremia is 146-150, moderate is 151-159, and severe is 160 or above. Symptoms include thirst, lethargy, irritability, confusion, and in severe cases, seizures and coma.
Chronic hypernatremia (over 48 hours or unknown duration) should be corrected at no more than 10-12 mEq/L per 24 hours. Acute hypernatremia (under 24 hours) can be corrected more rapidly. Too-rapid correction risks cerebral edema.
During chronic hypernatremia, brain cells accumulate intracellular osmoles to prevent dehydration. If sodium is corrected too quickly, water rushes into adapted brain cells causing cerebral edema, which can be fatal.
D5W (5% dextrose in water) provides pure free water once glucose is metabolized. Half-normal saline (0.45% NaCl) provides 50% free water. In mild cases, oral water intake may be sufficient. The choice depends on severity and clinical context.
No. The calculated deficit assumes no further water losses. In practice, insensible losses (600-900 mL/day), urinary losses, and any GI losses must be added to the deficit for accurate replacement planning.
Elderly patients with impaired thirst mechanism or limited water access, patients with diabetes insipidus, critically ill patients on mechanical ventilation, infants, and patients with osmotic diuresis (uncontrolled diabetes, mannitol therapy).
Normal serum sodium is 135-145 mEq/L. Values below 135 indicate hyponatremia; above 145 indicate hypernatremia. Sodium is tightly regulated and even small deviations can cause significant symptoms.
No. This calculator is specifically for hypernatremia (elevated sodium). Hyponatremia (low sodium) requires different calculations and treatment approaches, including potential fluid restriction rather than replacement.
The fractional body weight method provides a reasonable clinical estimate. Accuracy is limited in very obese patients (overestimates TBW) and very lean/muscular patients (may underestimate). It is sufficient for initial clinical calculations with serial monitoring.
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