Free Water Deficit Calculator — Hypernatremia Correction

Calculate free water deficit for hypernatremia from body weight, age/sex TBW fraction, and serum sodium. Includes the 0.5 mEq/L/hour safe correction limit.

July 6, 2026

Age Group / Sex

Serum Sodium (mEq/L)

Body Weight

Weight Unit

Free Water Deficit

6.00 L

Total body water (TBW) ≈ 42.0 L (70.0 kg × 0.6)

Clinical safety limits — read before use

This number is the water deficit only. It does not include ongoing losses (urine, insensible, GI) — a clinician must add those separately to get the total replacement volume.
Correction must not exceed 0.5 mEq/L per hour. Correcting too fast risks cerebral edema and osmotic demyelination syndrome (ODS).
Full correction is typically targeted over ~48 hours, not delivered all at once.
This is a clinical decision-support estimate for healthcare providers. Free water replacement in a hospital setting requires physician oversight — this tool is not for patient self-diagnosis or self-treatment.

Free Water Deficit (L) = TBW × [(Serum Na ÷ 140) − 1], where TBW = body weight (kg) × age/sex-specific fraction (child and adult male 0.6, adult female and elderly male 0.5, elderly female 0.45). 140 mEq/L is used as the target/normal serum sodium midpoint in the formula.

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Reference Values

Last verified: Jul 6, 2026

Category	Range	What It Means	Status
Child	0.6 × body weight (kg)	Children have a higher proportion of total body water than adults, similar to adult males on a per-kilogram basis.	Good
Adult male	0.6 × body weight (kg)	Standard TBW fraction used for adult males under 65 — higher lean muscle mass holds relatively more water than fat tissue.	Good
Adult female	0.5 × body weight (kg)	Standard TBW fraction used for adult females under 65 — higher average body fat percentage means relatively less total body water per kilogram than males.	Good
Elderly male	0.5 × body weight (kg)	TBW fraction declines with age due to loss of lean muscle mass, so elderly males use the same fraction as adult females.	Okay
Elderly female	0.45 × body weight (kg)	Lowest standard TBW fraction — reflects the combined effect of age-related lean mass loss and naturally higher body fat percentage.	Okay
Normal serum sodium range ★	135 – 145 mEq/L	Reference range for normal serum sodium. 140 mEq/L (the range's midpoint) is the target value used inside the free water deficit formula.	★ Best
Maximum safe correction rate	0.5 mEq/L per hour (≈8–10 mEq/L per 24 hours)	Correcting chronic hypernatremia faster than this risks cerebral edema and osmotic demyelination. Most protocols target full correction over roughly 48 hours.	Poor

Source: MDCalc Free Water Deficit calculator; Time of Care hypernatremia diagnosis and management reference; Adrogué HJ & Madias NE (2000) NEJM 'Hypernatremia' — standard TBW fraction and safe correction-rate conventions used in clinical practice.

Worked Examples

Adult Male With Severe Hypernatremia

Group: Adult male
Weight: 70 kg
Serum Na: 160 mEq/L

6.00 L free water deficit

TBW = 70 × 0.6 = 42 L. Deficit = 42 × (160/140 − 1) = 42 × 0.1429 = 6.00 L.

Adult Female With Moderate Hypernatremia

Group: Adult female
Weight: 60 kg
Serum Na: 155 mEq/L

3.21 L free water deficit

TBW = 60 × 0.5 = 30 L. Deficit = 30 × (155/140 − 1) = 30 × 0.1071 = 3.21 L.

Elderly Female With Mild Hypernatremia

Group: Elderly female
Weight: 55 kg
Serum Na: 150 mEq/L

1.77 L free water deficit

TBW = 55 × 0.45 = 24.75 L. Deficit = 24.75 × (150/140 − 1) = 24.75 × 0.0714 = 1.77 L.

Child With Hypernatremia

Group: Child
Weight: 20 kg
Serum Na: 158 mEq/L

1.54 L free water deficit

TBW = 20 × 0.6 = 12 L. Deficit = 12 × (158/140 − 1) = 12 × 0.1286 = 1.54 L.

Elderly Male With Severe Hypernatremia

Group: Elderly male
Weight: 80 kg
Serum Na: 165 mEq/L

7.14 L free water deficit

TBW = 80 × 0.5 = 40 L. Deficit = 40 × (165/140 − 1) = 40 × 0.1786 = 7.14 L.

How to Use This Calculator

1

Choose the age group and sex

Selects the correct total body water (TBW) fraction: 0.6 for children and adult males, 0.5 for adult females and elderly males, 0.45 for elderly females.
2

Enter body weight

In kilograms or pounds — the calculator converts automatically.
3

Enter the current serum sodium

In mEq/L, from a recent lab draw. The formula compares this against the 140 mEq/L target midpoint.
4

Read the deficit and the safety limits

The result shows the estimated free water deficit in liters alongside the maximum safe correction rate and what the number does not include.

What Each Value Means

Free Water Deficit (liters (L)): The estimated volume of free water (in liters) needed to bring an elevated serum sodium back down toward the normal target of 140 mEq/L, based on total body water and the degree of sodium elevation. Does not include ongoing losses or maintenance fluid needs.
Total Body Water (TBW) (liters (L)): The estimated total volume of water in the body, calculated as body weight multiplied by an age/sex-specific fraction. Used as the base volume the sodium concentration is diluted across.
Serum Sodium (mEq/L): The concentration of sodium in the blood, measured in mEq/L. Values above 145 mEq/L indicate hypernatremia; the normal reference range is roughly 135–145 mEq/L.

Frequently Asked Questions

What is the free water deficit formula?

Free Water Deficit (L) = TBW × [(Serum Na ÷ 140) − 1], where TBW (Total Body Water, in liters) equals body weight in kilograms multiplied by an age/sex-specific fraction: 0.6 for children and adult males, 0.5 for adult females and elderly males, and 0.45 for elderly females. The 140 mEq/L in the formula represents the target/normal serum sodium midpoint, not the patient's current value.

Why does the TBW fraction change by age and sex?

Total body water as a share of body weight depends mostly on lean muscle mass versus fat tissue, since muscle holds far more water than fat. Adult males and children carry proportionally more lean mass, so they use a 0.6 fraction. Adult females typically carry more body fat at the same weight, so they use 0.5. Aging further reduces lean mass in both sexes, which is why elderly males drop to 0.5 and elderly females — combining both effects — drop to 0.45, the lowest fraction of the group.

Why can't sodium be corrected all at once?

Correcting serum sodium faster than 0.5 mEq/L per hour (roughly 8–10 mEq/L over 24 hours) is a recognized safety limit because brain cells adapt to chronic hypernatremia by accumulating their own protective solutes. If plasma sodium falls too quickly, water rushes into those osmotically adapted brain cells, causing cerebral edema. Overly aggressive correction has also been linked to osmotic demyelination — historically associated with correcting low sodium too fast, but rapid swings in either direction around an adapted brain are the underlying risk. This is why replacement is spread out, typically targeting full correction over about 48 hours rather than delivering the calculated deficit in one infusion.

Does this number include maintenance fluids and ongoing losses?

No. The free water deficit formula only estimates the water needed to bring existing serum sodium back toward 140 mEq/L — it does not account for ongoing losses from urine output, insensible losses (skin and respiration), fever, or gastrointestinal losses, nor does it include a patient's normal maintenance fluid needs. A clinician calculating a full replacement plan adds these ongoing and maintenance components on top of the deficit shown here.

Is this calculator safe to use for self-treatment?

No. This tool is built for healthcare providers as a starting-point estimate during clinical decision-making, not for patients to self-diagnose or self-treat hypernatremia. Free water replacement — especially by IV — requires lab-confirmed serum sodium, an accurate weight, monitoring of correction rate, and physician oversight. If you or someone you know has symptoms of high sodium (confusion, extreme thirst, reduced urination, lethargy), seek medical care rather than relying on a calculator.