This free water deficit calculator estimates the water deficit in hypernatremia — deficit = total body water × (measured Na ÷ 140 − 1) — with total body water fractions by patient group and the correction-speed limits that make the number safe to use.

Fraction guide: adult man 0.6 · adult woman 0.5 · elderly man 0.5 · elderly woman 0.45 · child 0.6.

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Water Missing, Salt Concentrated: The Hypernatremia Ledger

The companion lab math is our Serum Osmolality Calculator. Part of our Health Calculators collection.

Worked Example (Verified)

  • 70 kg man, Na 160, target 140 (defaults): TBW = 70 × 0.6 = 42 L → deficit = 42 × (160/140 − 1) = 6.0 L — staged across days at safe correction rates, not delivered in a shift
  • The fraction does the silent work: the same sodium in a 60 kg elderly woman (× 0.45 = 27 L TBW) yields a 3.9 L deficit — identical lab value, one-third less water, because body composition is part of the formula
  • Target flexibility: correcting an acute Na of 160 toward 150 first (deficit to 150 = 42 × (160/150 − 1) = 2.8 L) is often the day-one goal; the target field exists because full correction is deliberately staged

Correction Discipline

  • Chronic vs acute is the safety fork: hypernatremia developed over > 48 hours (or unknown duration) corrects SLOWLY (≤ ~10 mmol/L/day); documented-acute salt loading is the rare exception where faster is tolerated.
  • The deficit formula counts only the water gap — ongoing urine/insensible losses continue during correction and must be added separately.
  • Hypovolemia outranks tonicity: restore perfusion with isotonic fluid first when the circulation is compromised, then address free water.
  • Route matters: enteral water is preferred when the gut works; IV D5W is the usual parenteral free-water source — rates set with the Adrogué-Madias change-in-Na estimate per litre.

Related Tools

Frequently Asked Questions

How is the free water deficit calculated?

Deficit (L) = total body water × (measured Na ÷ target Na − 1), where total body water = weight × fraction (0.6 adult men, 0.5 adult women, 0.5/0.45 elderly men/women, 0.6 children). Example: 70 kg man at Na 160 → 42 L × (160/140 − 1) = 6.0 L.

How fast should hypernatremia be corrected?

Slowly, when chronic or of unknown duration: no more than ~10 mmol/L per 24 hours (≈ 0.5 mmol/L/hr). The adapted brain accumulates osmoles; rapid water repletion swells it — cerebral edema is the complication the speed limit exists to prevent.

Why do body-water fractions differ by sex and age?

Muscle holds water; fat largely doesn’t. Higher average adiposity in women and age-related muscle loss (sarcopenia) shrink the water fraction of body weight — 0.6 in younger men down to 0.45 in elderly women — so identical sodium readings imply meaningfully different deficits.

Does the deficit include ongoing losses?

No — the formula is a snapshot of the existing gap. Fever, osmotic diuresis, diabetes insipidus and simple insensible losses keep spending water during correction; clinicians add estimated maintenance + ongoing losses to the prescription and recheck sodium frequently.

What fluids replace free water?

Enteral water when the gut works; intravenously, D5W (effectively free water once the dextrose is metabolized) or hypotonic saline depending on volume status. Isotonic fluids restore circulation first in hypovolemia — sequencing is a clinical decision.

Is this free water deficit calculator medical advice?

No — an educational implementation of the standard formula. Hypernatremia is a serious electrolyte disorder whose management requires laboratory monitoring and a treating clinician.