What is free water deficit?

Free water deficit is the estimated volume of pure (solute-free) water that is missing from the body, causing the serum sodium to rise above normal. Replacing this volume with hypotonic fluid restores sodium balance.

What formula does this calculator use?

It uses the Adrogue-Madias formula: Free Water Deficit (L) = Total Body Water × ((Serum Na / 140) − 1), where Total Body Water = weight (kg) × body water fraction (0.60 for adult males, 0.50 for adult females, 0.45 for elderly females, 0.50 for elderly males, 0.60 for children).

What is the target serum sodium used in the formula?

The calculator uses 140 mEq/L as the target (normal) serum sodium, which is the standard value used in the Adrogue-Madias formula.

How fast should the free water deficit be corrected?

Clinical guidelines recommend correcting hypernatremia no faster than 10–12 mEq/L per 24 hours to prevent cerebral edema. Chronic hypernatremia should be corrected even more slowly (8–10 mEq/L/24 h). This tool provides the total deficit; rate of correction must be determined by a clinician.

Is this tool a substitute for clinical judgment?

No. This tool is for educational and reference purposes only. Always verify calculations and individualize therapy based on the patient's clinical status, ongoing losses, and response to treatment.

What body water fractions are used?

Adult male: 60% (0.60), Adult female: 50% (0.50), Elderly male (>65): 50% (0.50), Elderly female (>65): 45% (0.45). These fractions reflect differences in lean body mass and adipose tissue.

Free Water Deficit Calculator

Name: Free Water Deficit Calculator
Availability: InStock
Author: Nham Vu

Estimate the free water deficit (in liters) for hypernatremia patients using body weight, sex, and serum sodium.

Patient Parameters

Body Weight

Sex

Male Female

Age Category

Current Serum Sodium (mEq/L)

Normal: 135–145 mEq/L. Hypernatremia: > 145 mEq/L.

Enter patient parameters and press Calculate.

Copied to clipboard!

Summary

Estimate the free water deficit (in liters) for hypernatremia patients using body weight, sex, and serum sodium.

How it works

Enter the patient's body weight in kilograms (or pounds — the tool converts automatically).
Select the patient's sex (male, female) and age category (adult or elderly/child) to determine the appropriate total body water fraction.
Enter the patient's current serum sodium in mEq/L (normal range is 135–145 mEq/L; hypernatremia is defined as > 145 mEq/L).
The tool applies the formula: Free Water Deficit = TBW × ((Na_actual / Na_normal) − 1), where TBW = weight × body water fraction.
Results are displayed in liters, alongside the estimated total body water and a correction rate guide.

Use cases

Estimating fluid replacement volume for hypernatremic patients in emergency or ICU settings.
Guiding oral or IV hypotonic fluid therapy for dehydrated patients.
Teaching medical students and nurses the Adrogue-Madias free water deficit formula.
Cross-checking manually calculated water deficits at the bedside.
Monitoring correction progress by recalculating after each sodium recheck.
Planning safe correction rates to avoid osmotic demyelination syndrome.

Frequently Asked Questions

Last updated: 2026-06-09 · Reviewed by Nham Vu

Free Water Deficit Calculator

Patient Parameters