Free Water Deficit Calculator

Free Water Deficit Calculator for Nephrology. The result is a volume, not a rate. Replace it gradually: for chronic hypernatremia (>48 h or unknown onset) do not lower serum Na faster than about 10 mEq/L per 24 h (many sources cite 0.5 mEq/L/h) to avoid cerebral edema and seizures; acute hypernatremia may be corrected faster. Add ongoing obligatory and insensible free-water losses (roughly 30-40 mL/kg/day plus urine output) on top of the calculated deficit, and recheck serum sodium every 4-6 hours because the formula systematically under-predicts the water needed when losses continue.

How this calculator works

The free water deficit estimates the liters of electrolyte-free water a hypernatremic patient must retain or receive to return serum sodium to a target (usually 140 mEq/L). It multiplies total body water (TBW) by the fractional sodium excess: FWD = TBW x [(measured Na / target Na) - 1], where TBW = body weight (kg) x a correction factor (0.6 in children and non-elderly men, 0.5 in women and elderly men, 0.45 in elderly women). Because the equation ignores ongoing renal and insensible losses, most clinicians pair it with the Adrogué-Madias infusate formula to set an hourly rate.

When to use this calculator

Use it for acute or chronic hypernatremia (Na >145 mEq/L) from pure water loss or hypotonic fluid loss, e.g. diabetes insipidus, unreplaced insensible/GI losses, or restricted water access in the elderly or ventilated. In hypovolemic hypernatremia, restore hemodynamics with isotonic saline first, then use the deficit to guide free-water replacement. It should NOT be used to dose sodium in hyponatremia, and it is unreliable in patients with large ongoing urine output (uncontrolled DI, osmotic diuresis) where losses dwarf the calculated deficit.

Inputs used

  • Measured sodium
  • Target sodium
  • Weight
  • Sex or total body water estimate
  • Age or body composition adjustment when used

Clinical interpretation

The result is a volume, not a rate. Replace it gradually: for chronic hypernatremia (>48 h or unknown onset) do not lower serum Na faster than about 10 mEq/L per 24 h (many sources cite 0.5 mEq/L/h) to avoid cerebral edema and seizures; acute hypernatremia may be corrected faster. Add ongoing obligatory and insensible free-water losses (roughly 30-40 mL/kg/day plus urine output) on top of the calculated deficit, and recheck serum sodium every 4-6 hours because the formula systematically under-predicts the water needed when losses continue.

Worked example

A 70-kg elderly man with serum Na 160 mEq/L: TBW = 70 x 0.5 = 35 L; FWD = 35 x [(160/140) - 1] = 35 x 0.143 = 5.0 L of free water to reach 140 mEq/L. To respect a safe correction limit of about 10 mEq/L per 24 h, aim to close roughly half this deficit (drop to ~150 mEq/L) over the first day, i.e. give ~2.5 L of the deficit as electrolyte-free water plus ongoing losses over 24 hours.

Limitations and safety notes

The deficit assumes a static patient: it excludes ongoing urinary and insensible losses, so in diabetes insipidus or osmotic diuresis it substantially underestimates requirements. The TBW correction factors are population estimates that misfire in obesity, cachexia, ascites, or large third-space shifts. Validation data show the Adrogué-Madias framework predicts 12-24 h sodium change with only modest accuracy (mean achieved Na often exceeded predicted by 2-6 mEq/L), so it is a starting estimate, not a substitute for serial sodium measurement. It does not account for the sodium and potassium content of the chosen infusate.

Frequently asked questions

Which correction factor should I use for total body water?

Use 0.6 for children and non-elderly men, 0.5 for women and elderly men, and 0.45 for elderly women. These reflect the lower fractional body water of women and older adults; using 0.6 for everyone overestimates the deficit in the elderly.

Does the free water deficit tell me the infusion rate?

No. It gives only the total volume needed. You must divide replacement over a safe interval (limiting the fall in Na to about 10 mEq/L/24 h in chronic hypernatremia) and add ongoing insensible and urinary losses separately, often using the Adrogué-Madias infusate equation.

Why did my patient's sodium not fall as much as the formula predicted?

Because the deficit ignores continued free-water losses. In diabetes insipidus, osmotic diuresis, or ongoing GI losses, water is still being lost during treatment, so the true requirement exceeds the calculated deficit. Recheck sodium every 4-6 hours and titrate.

Should I correct hypernatremia with the deficit before or after fluid resuscitation?

After. If the patient is hypovolemic, restore circulating volume with isotonic fluid first, then use the free water deficit to guide the hypotonic free-water replacement needed to lower serum sodium.

References

  • Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-9. PMID: 10816188.
  • Liamis G, Kalogirou M, Saugos V, Elisaf M. Therapeutic approach in patients with dysnatraemias. Nephrol Dial Transplant. 2006;21(6):1564-9. PMID: 16449285.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: March 17, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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