Free Water Deficit Calculator
Calculate free water deficit and sodium correction volume for hypernatremia treatment and fluid management.
Enter serum sodium levels, body weight, and patient type to calculate the free water deficit and determine appropriate correction volumes for hypernatremia.
Free Water Deficit Calculator
Calculate free water deficit and sodium correction volume for hypernatremia treatment and fluid management.
About the Free Water Deficit Calculator
Hypernatremia — defined as a serum sodium concentration above 145 mEq/L — indicates a deficit of free water relative to total body sodium. This calculator estimates the volume of free water needed to restore sodium to a target concentration, using the patient's total body water (TBW) as the foundation for the calculation.
The free water deficit formula is: FWD (L) = TBW × ((current Na / desired Na) − 1). TBW is estimated as a fraction of body weight depending on age and sex. Adult males use a factor of 0.6 (60% of body weight), adult females use 0.5 (50%), elderly males use 0.5, elderly females use 0.45, and children use 0.6. These factors reflect known differences in muscle mass and body composition, since muscle contains more water than fat.
Severe hypernatremia (Na+ >160 mEq/L) is a medical emergency with significant morbidity and mortality. The most important principle of treatment is to correct sodium slowly — rapid correction can cause cerebral edema, seizures, and cerebral herniation. Current guidelines recommend reducing serum sodium by no more than 10–12 mEq/L per 24 hours, or approximately 0.5 mEq/L per hour in acute cases.
The choice of fluid for correction depends on the clinical context. Hypotonic fluids such as 5% dextrose in water (D5W), half-normal saline (0.45% NaCl), or quarter-normal saline are typically used. The infusion rate is determined by dividing the calculated free water deficit by the desired correction time in hours, then adjusting for the tonicity of the solution being administered.
Free water deficit alone does not account for ongoing losses or maintenance requirements, which must be added to the calculated volume for a complete fluid prescription. Patients with diabetes insipidus, osmotic diuresis, or continued insensible losses will require additional supplementation beyond the calculated deficit replacement.
This calculator provides an estimate for educational and clinical decision-support purposes. All hypernatremia treatment decisions should be made by qualified healthcare professionals with real-time monitoring of serum electrolytes and clinical status.
Free water deficit examples
Click any example button below to load these clinical scenarios.
| Patient Parameters | Free Water Deficit | Clinical Context |
|---|---|---|
| Na 165 → 140, 70 kg adult male | FWD = 12.5 L (TBW 42 L) | Severe hypernatremia — requires careful slow correction over 48–72 hours |
| Na 155 → 140, 65 kg adult female | FWD = 4.9 L (TBW 32.5 L) | Moderate hypernatremia — correct at ≤0.5 mEq/L per hour |
| Na 152 → 140, 20 kg child | FWD = 2.0 L (TBW 12 L) | Pediatric case — use weight-based dosing with careful electrolyte monitoring |
How to use the free water deficit calculator
- Enter the patient's current serum sodium level in mEq/L from the most recent lab result.
- Enter the desired target sodium level (typically 140 mEq/L) or the level recommended by the treating physician.
- Enter the patient's body weight in kilograms and select the appropriate patient type for TBW estimation.
- Click Calculate to see the free water deficit in liters plus suggested correction volumes for 24-hour and 48-hour schedules.
- Use the correction volume as a starting point only — adjust based on repeat sodium measurements every 4–6 hours.
Free water deficit calculator FAQ
What is free water deficit?
Free water deficit is the volume of pure water needed to dilute elevated serum sodium back to a target level. It quantifies how much water the body is short relative to its sodium content, providing a starting estimate for fluid replacement in hypernatremia treatment.
How fast should hypernatremia be corrected?
Current guidelines recommend correcting serum sodium by no more than 10–12 mEq/L per 24 hours. Rapid correction — especially in chronic hypernatremia — can cause cerebral edema, seizures, and brain herniation because brain cells accumulate osmolytes to compensate. Always monitor sodium every 4–6 hours during active correction.
What TBW factor is used for each patient type?
Total body water is estimated as a fraction of body weight: 60% for adult males and children, 50% for adult females and elderly males, and 45% for elderly females. These factors reflect the higher water content of muscle compared to fat, and the progressive decrease in lean mass with aging.
Does the calculated deficit include ongoing losses?
No. The free water deficit formula only estimates the replacement needed to correct the existing sodium elevation. Ongoing losses from urine, perspiration, respiratory evaporation, or diarrhea must be added to the calculated deficit to determine the full fluid requirement. Patients with diabetes insipidus may need substantially more.
What fluids are used to replace free water?
Commonly used hypotonic fluids include 5% dextrose in water (D5W, which is electrolyte-free), 0.45% sodium chloride (half-normal saline), or 0.225% sodium chloride (quarter-normal saline). The choice depends on the clinical setting, hemodynamic status, and any co-existing electrolyte abnormalities.
Is this calculator a substitute for clinical judgment?
No. Free water deficit calculation is a starting point for fluid prescription, not a complete treatment plan. Actual correction rates must be individualized based on frequent sodium monitoring, urine output, hemodynamic status, and the underlying cause of hypernatremia. Always involve a physician in acute hypernatremia management.