- Enter the patient's weight in kg. Use actual body weight (not ideal body weight).
- Enter the current serum sodium in mEq/L. The calculator requires Na⁺ > 140 mEq/L to be applicable; a value ≤ 140 will trigger a warning.
- Select the sex and age category. This determines the total body water (TBW) fraction: male 0.60, female 0.50, elderly male 0.50, elderly female 0.45. Lean patients have higher TBW fractions; obese patients have lower fractions (the default categories approximate average composition).
- Click Calculate or values update on input. The results show: estimated TBW in liters, the free water deficit in liters, safe volume to give per 24 hours (half the deficit), and a correction rate guidance note.
- Add calculated insensible losses (~500–800 mL/day) and measured urine output to obtain total daily fluid requirement. Adjust the infusion rate every 4–6 hours based on repeat sodium levels.
All computation runs in your browser; no values are stored or transmitted.
When to Use
Use this tool when a patient has hypernatremia (serum Na⁺ > 145 mEq/L) and you need to estimate the volume of free water required to return sodium to the normal range, as well as the safe rate of correction to prevent cerebral edema from too-rapid sodium reduction.
Appropriate population
Any patient with Na⁺ > 145 mEq/L and an established or estimated weight. Most commonly: (1) elderly patients with insensible water loss, impaired thirst, or functional inability to drink; (2) patients with central or nephrogenic diabetes insipidus; (3) patients receiving hypertonic enteral feeds without sufficient free water; (4) neonates after inadequate fluid intake. The formula applies regardless of the cause of hypernatremia — it quantifies the deficit, not the rate of ongoing losses.
Critical limitation: ongoing losses must be added
The free water deficit formula calculates only the volume needed to correct the existing elevation — it does not account for ongoing insensible losses, urine output, or ongoing pathological free water losses (e.g., 10–20 L/day in central DI). In those settings, add estimated ongoing losses to the calculated deficit and re-check sodium every 4–6 hours, adjusting the infusion rate accordingly.
Pearls & Pitfalls
Ongoing losses are the hidden variable
The free water deficit formula gives a static snapshot of the electrolyte deficit. In central DI or nephrogenic DI with very high urine output, the hourly free water loss can vastly exceed the calculated deficit. Always measure urine output quantitatively and add it to the replacement volume. In practice this means reassessing and recomputing the plan at 4–6 hour intervals rather than following a single overnight rate.
Route matters — enteral free water is preferred
When the gastrointestinal tract is functional, oral free water or water via nasogastric tube is the safest and most physiological replacement. Intravenous D5W requires glucose monitoring and can cause hyperglycemia with subsequent osmotic diuresis, partially counteracting the correction. For volume-depleted hypernatremic patients, give an initial bolus of isotonic saline (for hemodynamic resuscitation), then transition to hypotonic fluids once circulating volume is restored.
Pitfalls
(1) Never correct hypernatremia faster than 10 mEq/L in 24 hours — cerebral edema has been reported, particularly in children and in adults with acute rapid rises in sodium. (2) The TBW fraction defaults in this calculator are estimates; obese patients have a lower fraction (use 0.5 for obese males) and very muscular patients have a higher fraction. When in doubt, use a lower fraction to avoid under-estimating the deficit. (3) Hypernatremia from salt poisoning (intake of hypertonic NaCl or enteral feeds) requires a different approach — the excess sodium must be removed (via furosemide + free water replacement or dialysis) rather than simply providing free water. (4) Do not use free water deficit calculation in acute hypernatremia of known onset < 24 hours — the brain has not had time to generate idiogenic osmoles, so a faster initial correction rate (up to 1 mEq/L/hour) may be appropriate.
Why Use It
Hypernatremia causes cerebral cell dehydration and shrinkage. Brain cells adapt by generating intracellular osmoles (idiogenic osmoles or organic osmolytes) over 24–48 hours to partially restore brain volume. If the hypernatremia is corrected too rapidly, brain cells take up the incoming free water faster than they can clear their accumulated idiogenic osmoles, causing cerebral edema, herniation, and death — particularly in children and in adults with acute severe hypernatremia.
The standard safe correction rate is a maximum reduction of 8–10 mEq/L of serum sodium per 24 hours (approximately 0.5 mEq/L/hour), with the full deficit corrected over 48 hours or longer for severe or chronic hypernatremia. This calculator provides the total volume to give and the approximate volume per 24 hours — the final infusion rate must then be adjusted based on serial sodium measurements every 4–6 hours.
Free Water Deficit Calculator — Hypernatremia
Enter the patient's weight, current serum sodium, and sex/age category (to select the appropriate total body water fraction). Results update automatically on input.
⚕ This calculator estimates the water deficit only. Add estimated ongoing losses (insensible, urinary, GI) to determine total fluid requirements. Recheck Na⁺ every 4–6 h during correction and adjust rate to maintain a reduction of no more than 8–10 mEq/L per 24 h.
Next Steps
Use the result to support — not replace — clinical judgment.
- Interpret the value against the targets shown in the calculator and the Evidence section below, in the context of the full clinical picture.
- Trend serial measurements rather than acting on a single result; confirm abnormal or unexpected values before changing management.
- Apply the relevant KDIGO / specialty-guideline threshold and document the indication.
- Escalate or refer to nephrology when results are out of range, rapidly changing, or discordant with the clinical picture — and discuss the implications with the patient.
Evidence & References
Formula & Equations
| Quantity | Equation |
|---|---|
| Total body water (TBW, L) | Weight (kg) × TBW fraction (0.60 / 0.50 / 0.45) |
| Free water deficit (L) | TBW × (current Na⁺ / 140 − 1) |
| Safe volume per 24 h (L) | Deficit ÷ 2 (targeting correction over 48 h; never exceed 8–10 mEq/L/24 h) |
TBW fractions by sex and age
| Category | TBW fraction | Rationale |
|---|---|---|
| Adult male | 0.60 | ~60% of body weight is water in lean males |
| Adult female | 0.50 | Higher adipose tissue reduces TBW fraction |
| Elderly male (≥ 65 years) | 0.50 | Loss of lean mass with aging reduces TBW |
| Elderly female (≥ 65 years) | 0.45 | Combined effect of sex and age on body composition |
Fluid choice for correction
| Fluid | Free water content | Use when |
|---|---|---|
| Enteral free water (PO / NGT) | 100% | Preferred; gut intact; not hemodynamically compromised |
| D5W (5% dextrose in water) | ~100% | IV when enteral route not feasible; watch glucose |
| 0.45% NaCl (half-normal saline) | ~50% | Moderate hypernatremia with volume depletion |
| 0.9% NaCl (normal saline) | 0% | For hypovolemic resuscitation only; will not lower Na⁺ |
The formula above (Adrogué–Madias, 2000) derives from the conservation of total body solute: Current Na⁺ × Current TBW = 140 × Desired TBW. The difference in TBW (desired − current) is the free water deficit.
Evidence & References
The free water deficit formula and the safe correction rate limit for hypernatremia are most completely described in the 2000 NEJM review by Adrogué and Madias. The maximum correction rate of 10 mEq/L per 24 hours is based on case series linking faster correction to cerebral edema, particularly in pediatric patients with acute hypernatremia and in adults who develop hypernatremia rapidly. The safe rate is more lenient than the corresponding limit for hyponatremia because the mechanism of harm (cerebral edema from idiogenic osmole clearance lag) is less consistent in adults. Serial sodium monitoring every 4–6 hours is the gold standard for ensuring the calculated rate matches the actual correction being achieved.
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493–1499.
- Sterns RH. Disorders of plasma sodium — causes, consequences, and correction. N Engl J Med. 2015;372(1):55–65.
- Mange K, Matsuura D, Cizman B, et al. Language guiding therapy: the case of dehydration versus volume depletion. Ann Intern Med. 1997;127(9):848–853.
