Free Water Deficit Calculator
Enter patient weight, sex, and serum sodium to calculate free water deficit and a safe rehydration target.
Patient Parameters
Normal range: 135–145 mEq/L. Hypernatremia: > 145 mEq/L.
Results
Enter values and click Calculate.
Total Body Water (TBW)
weight × TBW fraction
Free Water Deficit
TBW × (Na / 140 − 1)
Suggested Rate (24 h)
½ deficit over 24 h (chronic)
Suggested Rate (48 h)
Full deficit over 48 h (chronic)
Formula
Clinical note: These figures estimate the water deficit from sodium elevation only. Add ongoing losses (insensible, urine, GI) to your total replacement plan. Correct chronic hypernatremia no faster than 10–12 mEq/L per 24 h. This tool is for educational use — always verify with clinical judgment.
Summary
Enter patient weight, sex, and serum sodium to calculate free water deficit and a safe rehydration target.
How it works
- Enter patient weight in kilograms (or pounds — the tool converts automatically).
- Select biological sex to apply the correct TBW fraction (0.6 for male, 0.5 for female).
- Enter the measured serum sodium in mEq/L.
- The tool computes TBW = weight × TBW fraction, then FWD = TBW × (Na / 140 − 1).
- Results show free water deficit in liters and a suggested correction rate (½ deficit over 24 h).
Use cases
- Estimating fluid replacement volume for hypernatremic patients.
- Guiding initial resuscitation orders in the ICU or ED.
- Teaching medical students the TBW-based FWD formula.
- Quickly cross-checking a bedside mental calculation.
- Planning gradual sodium correction to avoid cerebral edema.