Free Water Deficit

What This Calculator Measures

Estimates the free water deficit in a patient with hypernatremia — the approximate volume of free water needed to bring the serum sodium from its current value down to a chosen target — using total body water estimated as 0.6 × body weight.

How to Use

1. Enter the patient's weight, current serum sodium, and the desired target sodium.
2. The result is an estimate of the free water deficit in liters; it represents only the water deficit and does not account for ongoing losses.
3. Review the calculator's severity band as context, but interpret the value together with the clinical picture.
4. Correction-rate concepts (for example, limiting correction to roughly 0.5 mmol/L per hour or 10–12 mmol/L per day) are educational background for planning and are not part of the calculator output.

How the Calculation Works

Free water deficit = total body water x (current sodium / desired sodium - 1), where total body water is estimated as 0.6 x weight (kg). Negative values are reported as zero.

Variables Explained

Interpretation Guide

The calculator reports the deficit in liters and classifies it as no deficit (≤0 L), mild (0.1–3 L), moderate (3.1–7 L), or severe (>7 L). The value is an estimate: it assumes total body water is 0.6 × weight, ignores ongoing water losses, and does not reflect the underlying cause of hypernatremia. Management depends on volume status (hypovolemic, euvolemic, or hypervolemic) and on concurrent losses, and the deficit should generally be corrected gradually rather than replaced as a single bolus.

Clinical Significance

Estimating the free water deficit gives clinicians a quantitative starting point for planning the gradual correction of hypernatremia and anticipating the volume of water replacement required, while emphasizing that the estimate must be adjusted for ongoing losses and the patient's volume status.

When It Is Useful

• Adults with hypernatremia where a target serum sodium has been defined
• Estimating the volume of free water needed to plan gradual correction
• Educational review of the water deficit implied by a given elevation in serum sodium

When It Is Not Useful

• As a complete fluid management plan — ongoing losses and maintenance requirements are not included
• In children and neonates without age-appropriate total body water assumptions
• Without assessment of volume status and the underlying cause of hypernatremia

Common Mistakes

• Applying the 0.6 total body water factor to a woman, when about 0.5 times weight is the commonly used assumption.
• Using the calculator in neonates or children without age-appropriate total body water assumptions.
• Treating the one-time deficit as a fixed infusion order rather than an estimate to be revised by serial sodium measurements.

Limitations

• Assumes total body water is 0.6 × weight; the calculator does not apply the lower fraction (~0.5 × weight) commonly used in women.
• Represents the deficit at a single point in time and does not model ongoing GI, renal, or insensible losses.
• Does not consider the cause of hypernatremia (e.g., diabetes insipidus, osmotic diuresis, or inadequate intake).
• The result is an estimate for planning gradual correction, not a prescribed infusion order.

Worked Example

An 80 kg patient with serum sodium 160 mmol/L is being evaluated; a target sodium of 140 mmol/L is selected.

Expected Result: Free water deficit = 0.6 × 80 × (160/140 − 1) ≈ 6.9 L — reported by the calculator as a moderate free water deficit.

Evidence & References

Source:

Nephrology / Fluids & Electrolytes

Content updated:

2026-08

1. Adrogue HJ, Madias NE. N Engl J Med. 2000;342(21):1493-1499.

References

1. Adrogue HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(21):1493-1499. (Review)
2. Sterns RH. Disorders of plasma sodium. N Engl J Med. 2015;372(1):55-65. (Review)

Frequently Asked Questions