How do you calculate free water deficit?

Free water deficit = TBW × (Serum Na / 140 − 1), where TBW (total body water) = weight (kg) × correction factor (0.6 for young men, 0.5 for young women/elderly men, 0.45 for elderly women). For example, a 70 kg elderly man with Na 155: Deficit = (70 × 0.5) × (155/140 − 1) = 3.75 L.

How fast should hypernatremia be corrected?

Sodium should be lowered by no more than 10–12 mEq/L per 24 hours to avoid cerebral edema. For chronic hypernatremia (>48 hours or unknown duration), aim for 0.5 mEq/L per hour. For acute hypernatremia (<24 hours), faster correction at 1 mEq/L per hour may be safe. Monitor serum sodium every 4–6 hours during correction.

What fluids are used to correct hypernatremia?

D5W (free water) is the purest replacement but must be given slowly to avoid hyperglycemia. Hypotonic saline (0.45% NaCl) is commonly used. Enteral free water (via NG tube or oral) is preferred when possible. The calculated deficit provides an estimate — ongoing losses (insensible, urine) must also be replaced.

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Free Water Deficit Calculator

Estimate the free water deficit for hypernatremia correction. Uses the Adrogue-Madias approach with total body water estimation.

Inputs

Sex

Weight (kg)

Current Serum Sodium (mEq/L)

Target Sodium (mEq/L)

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Enter values to calculate free water deficit.

⚠️ Important: Correct sodium no faster than 10–12 mEq/L per 24 hours (chronic hypernatremia) or 1 mEq/L per hour (acute). Rapid correction risks cerebral edema.

Disclaimer: For educational purposes only. Not a substitute for clinical judgment.

When to use

Use this calculator to estimate the volume of free water needed to correct hypernatremia. This is a starting point — ongoing losses (insensible, renal, GI) must also be replaced.

Clinical pearls

Total Body Water (TBW) is estimated as a fraction of weight: 0.6 (young male), 0.5 (young female), 0.5 (elderly male), 0.45 (elderly female).
The formula: FWD = TBW × [(Current Na / Target Na) − 1].
This calculates the deficit only — you must also replace ongoing losses (urine, insensible, GI).
Recheck sodium every 4–6 hours and adjust the infusion rate accordingly.

References

Adrogue HJ, Madias NE. Hypernatremia. NEJM. 2000;342(20):1493-1499.
Sterns RH. Disorders of plasma sodium. NEJM. 2015;372(1):55-65.

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About This Tool

What Is Free Water Deficit?

Free water deficit estimates the volume of electrolyte-free water needed to lower serum sodium to a target level in hypernatremic patients. Hypernatremia (Na > 145 mEq/L) reflects a deficit of water relative to sodium and is most commonly caused by inadequate water intake, diabetes insipidus, or excessive insensible losses.

The Formula

Free Water Deficit (L) = TBW × [(Current Na ÷ Target Na) − 1]

Where TBW = body weight × TBW fraction (0.6 for young men, 0.5 for young women/elderly men, 0.45 for elderly women). This formula calculates only the existing deficit — ongoing free water losses must be added to the replacement volume.

🔑 Clinical Pearls

Chronic hypernatremia (> 48h): correct no faster than 10–12 mEq/L per 24 hours to avoid cerebral edema.
Acute hypernatremia (< 48h): can correct faster, up to 1–2 mEq/L per hour.
D5W or 0.45% NaCl are common replacement fluids. D5W provides ~1 L of free water per liter; 0.45% NaCl provides ~0.5 L.
Don't forget to account for ongoing losses — the deficit formula is a snapshot, not a complete treatment plan.

Key References

Adrogue HJ, Madias NE. Hypernatremia. NEJM. 2000;342(20):1493-1499.
Sterns RH. Disorders of plasma sodium. NEJM. 2015;372(1):55-65.

Formula last verified: February 2026