Free Water Deficit Calculator (Hypernatremia)

Your details

Total body water as a fraction of weight varies by age and sex. Select the category that best describes your patient.
Current body weight. Use actual (not ideal) body weight.
kg
Measured serum sodium. Hypernatremia is defined as sodium above 145 mEq/L.
mEq/L
Goal sodium level. Most guidelines recommend 140-145 mEq/L. Do not lower below 140 mEq/L rapidly.
mEq/L
Chronic hypernatremia (>48 h duration) should be corrected slowly - no faster than 10 mEq/L per 24 hours - to avoid cerebral edema.
Free water deficitModerate deficit (1-3 L)
2.9L

Volume of electrolyte-free water needed to correct serum sodium to target

Total body water42L
Sodium to correct10mEq/L
Infusion rate60.3mL/h
Max safe Na drop / 24 h10mEq/L
2.9 L
Mild<1Moderate1-3Severe3+

Free water deficit is 2.90 L (moderate) - infuse at 60.3 mL/h over 48 hours.

  • Total body water is estimated at 42.0 L (60% of body weight for this patient category).
  • The free water deficit is 2.90 L - the volume of electrolyte-free water (e.g., D5W or oral/enteral water) needed to bring sodium from 155 to 145 mEq/L.
  • A 48-hour correction window is appropriate for this 10 mEq/L deficit (within the 10 mEq/L per 24 h safety limit).
  • This formula calculates only the existing static deficit. Add estimated ongoing free water losses (insensible, urinary, and GI losses) to the total replacement volume.

Next stepRecheck serum sodium every 4-6 hours during correction. If acute hypernatremia (onset <48 h), faster correction may be appropriate; if chronic or unknown duration, cap correction at 10 mEq/L per 24 hours.

 = Powered by OnlyCalculators