Renal & Electrolytes

Free Water Deficit Calculator

Water needed to bring a high sodium back to target, from total body water; replace it slowly.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

Turn on JavaScript to use this calculator, or open it in the web app.

Formula

Deficit = Total Body Water × ((Measured Na / Desired Na) - 1)

How it is used

• Total Body Water (TBW) factor: 0.6 for adult males, 0.5 for adult females and elderly males, 0.45 for elderly females.
• Do not correct sodium faster than 10-12 mEq/L per 24 hours to avoid cerebral edema.

Same formula as the app. Open it in the web app.

Related one-pagers

• Electrolyte ImbalancesFree water deficit = TBW × (Na ÷ 140 − 1); lower chronic hypernatremia about 10 mEq/L a day.
• ABG Interpretation in 6 Steps
• CRRT (Continuous Renal Replacement)

Related calculators

• Anion Gap
• BUN/Cr Ratio
• Corrected Calcium
• Corrected Sodium (Hyperglycemia)
• Creatinine Clearance (Cockcroft-Gault)
• CRRT Effluent Dose (mL/kg/h)
• Fractional Excretion of Sodium (FENa)
• Fractional Excretion of Urea (FEUrea)

For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your institution’s protocols. Not a substitute for clinical judgment.