Free Water Deficit Calculator

Sex	TBW factor
Male	0.6 L/kg
Female	0.5 L/kg

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Walk-through

How to Use This Calculator

3 steps

1

Enter weight, sex, and current sodium

Enter the patient's weight in kilograms, biological sex, and current serum sodium in mEq/L on the Water Deficit tab. Target sodium defaults to 140 mEq/L but can be adjusted if a different clinical target applies.

2

Read the free water deficit

The result card shows the free water deficit in liters — the volume of free water estimated to bring serum sodium back down to the target. It updates instantly as you change any input.

3

Check the TBW breakdown and replacement notes

Switch to the TBW tab to see how total body water was calculated from weight and sex, and to the Replacement Plan tab for general pacing notes — actual replacement rate is a clinical judgment call, not computed here.

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Reference

Formula & Methodology

2 formulas

Free water deficit

Deficit (L) = TBW × ((current Na / target Na) − 1)

Current sodium (Na) and target sodium are entered in mEq/L; the target defaults to 140 mEq/L, the standard midpoint of the normal range. TBW is total body water in liters. The result is in liters and matches MDCalc's "Free Water Deficit in Hypernatremia" tool.

Total body water (TBW)

TBW (L) = weight (kg) × TBW factor

The TBW factor is a standard adult estimate: 0.6 L/kg for males and 0.5 L/kg for females, reflecting the higher proportion of body water in lean tissue. These factors do not account for pediatric or elderly physiology.

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High YMYL

Trust, Methodology & Sources

Reviewed by Calculover Clinical Team · Updated 2026-07-22 · 2 sources

Editorial accountability

Author: Calculover Editorial Team - Clinical content team

Owner: Calculover Editorial - Editorial oversight

Reviewer: Calculover Clinical Team - Clinical reviewer

Last reviewed: 2026-07-22

Last verified: 2026-07-22

Data effective: 2026-07-22

Methodology

Implements the free water deficit formula as used by MDCalc's "Free Water Deficit in Hypernatremia": Deficit (L) = TBW × ((current Na / target Na) − 1), with TBW estimated as weight (kg) × 0.6 (male) or × 0.5 (female).

Assumption: Total body water is estimated using standard adult TBW factors (0.6 L/kg male, 0.5 L/kg female) — pediatric, elderly, and obese patients may need adjusted factors.

Assumption: Target sodium defaults to 140 mEq/L but can be changed to reflect a different clinical target.

Assumption: The formula assumes current sodium is above target (hypernatremia); results are not clinically meaningful when sodium is at or below target.

Limitations & guidance

This calculator reports the total water deficit only, not a replacement rate or infusion schedule — pacing is a clinical decision, and rapid correction of chronic hypernatremia risks cerebral edema.

Ongoing insensible or urinary water losses are not included and should be estimated and added separately.

Results are for clinical decision support and education only and do not replace a treating clinician's full assessment.

Professional guidance: This calculator is for clinical decision support and educational purposes only and is not a substitute for professional medical judgment, diagnosis, or treatment. Always consult a qualified clinician for patient care decisions.

Primary sources

MDCalc. Free Water Deficit in Hypernatremia.

Adrogué HJ, Madias NE. "Hypernatremia." N Engl J Med. 2000;342(20):1493-1499.

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Glossary

Key Terms Explained

6 terms

Free water deficit The estimated volume of free (electrolyte-free) water needed to correct hypernatremia and bring serum sodium back down to a target concentration, calculated as TBW × ((current Na / target Na) − 1).

Hypernatremia A serum sodium concentration above the normal range (typically above roughly 145 mEq/L), most often caused by a relative water deficit from inadequate intake or excessive water loss.

Total body water (TBW) The total volume of water in the body, estimated from weight and sex using a standard fraction (0.6 L/kg for males, 0.5 L/kg for females in adults). Sodium is distributed throughout this water volume.

Water replacement The clinical process of administering free water (oral or IV) to correct a water deficit. Replacement pacing — how quickly to give the calculated deficit — is a separate clinical decision not computed by this calculator.

Dehydration A state of reduced total body water. Hypernatremic dehydration specifically implies water loss in excess of sodium loss, raising the serum sodium concentration.

TBW factor The fraction of body weight assumed to be water, used to estimate total body water from weight. This calculator uses standard adult values of 0.6 (male) and 0.5 (female); pediatric and elderly patients may need adjusted factors.

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Scenarios

Real-World Examples

3 worked examples

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Hypernatremic dehydration

70 kg male, sodium 158 mEq/L

Weight 70 kg Sex Male Current sodium 158 mEq/L Target sodium 140 mEq/L

TBW = 70 × 0.6 = 42 L. Deficit = 42 × ((158 / 140) − 1) ≈ 5.4 L. This patient needs roughly 5.4 L of free water to correct sodium back to 140 mEq/L, typically replaced gradually rather than all at once.

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TBW factor comparison

Same weight and sodium, different sex

Weight 70 kg Current sodium 155 mEq/L Male TBW 42 L → deficit ≈ 4.5 L Female TBW 35 L → deficit ≈ 3.8 L

At the same weight and sodium, a lower TBW factor (female, 0.5 vs. male 0.6) produces a smaller total body water estimate and therefore a smaller calculated free water deficit — sex materially changes the result.

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Replacement pacing note

5.4 L deficit, chronic hypernatremia

Free water deficit 5.4 L Onset Chronic (>48h)

For chronic hypernatremia, correction is typically spread over 48 hours or more — replacing roughly half the deficit in the first 24 hours — to avoid cerebral edema from overly rapid correction. This calculator reports only the deficit, not a rate; pacing is a clinical decision.

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Deep Dive

Understanding the Free Water Deficit

3 chapters · 4 min read

The free water deficit estimates how much free water is needed to correct hypernatremia — an elevated serum sodium — by bringing the concentration back down to a normal target. This calculator computes it from body weight, sex, and current versus target sodium, matching the formula used by MDCalc's widely referenced clinical tool.

How the free water deficit works

Serum sodium concentration reflects the ratio of sodium to total body water. Hypernatremia most commonly results from a relative loss of water rather than a gain of sodium, so correcting it means replacing the missing water. The free water deficit formula, Deficit = TBW × ((current Na / target Na) − 1), estimates the volume of water needed to dilute sodium back to the target concentration, assuming total body sodium stays constant.

Total body water (TBW) is estimated from body weight using standard adult fractions — roughly 0.6 L/kg for males and 0.5 L/kg for females — reflecting typical differences in lean body mass and fat distribution between sexes.

Inputs and what they mean

Weight is entered in kilograms and drives the total body water estimate. Sex selects the TBW factor (0.6 male, 0.5 female). Current sodium is the patient's measured serum sodium in mEq/L, and target sodium defaults to 140 mEq/L — the standard midpoint of the normal range — but can be adjusted if a clinician is targeting a different value.

The formula assumes hypernatremia (current sodium above target). If current sodium is at or below target, the calculated "deficit" is zero or negative and is not clinically meaningful for this formula.

Limits and edge cases

This calculator reports only the free water deficit — the total volume needed — not a replacement rate or infusion schedule. Correcting hypernatremia too quickly, particularly when it developed gradually (chronic hypernatremia), risks cerebral edema; a common clinical approach is to replace roughly half the deficit over the first 24 hours and the rest over the following 24 hours, adjusting for the individual case.

The TBW factors used here (0.6/0.5) are standard adult estimates and do not account for pediatric, elderly, obese, or edematous patients, whose body water fraction can differ meaningfully. The calculator also does not include ongoing insensible or urinary water losses, which should be estimated separately and added to the total replacement volume. This tool is for clinical decision support and education only, not a substitute for a treating clinician's full assessment.

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Questions

Frequently Asked Questions

6 questions

What is the free water deficit formula?+

Free water deficit (L) = TBW × ((current sodium / 140) − 1), where TBW is total body water in liters. This estimates the volume of free water needed to bring serum sodium back to the normal 140 mEq/L target.

What TBW factor should I use?+

This calculator uses standard adult total body water factors: 0.6 L/kg for males and 0.5 L/kg for females. These are simplified estimates — pediatric and elderly patients may need adjusted factors.

What replacement rate should I use?+

Replacement rate is a matter of clinical judgment and is out of scope for this calculator. In general, replace roughly half the deficit over the first 24 hours and the remainder over the following 24 hours, correcting chronic hypernatremia slowly to avoid cerebral edema.

What units does this calculator use?+

Weight is entered in kilograms, sodium values in mEq/L, and both the free water deficit and total body water results are in liters.

Why is 140 mEq/L the target sodium?+

140 mEq/L is the standard midpoint of the normal serum sodium reference range and the conventional target used in the free water deficit formula. You can adjust the target sodium field if a different clinical target applies.

Does this include ongoing fluid losses?+

No. This calculator estimates only the existing free water deficit. Ongoing insensible losses, urinary losses, or other fluid losses should be estimated separately and added to the total replacement volume.

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