Hyponatraemia Sodium Deficit
Calculate the estimated sodium deficit based on total body water and the desired increase in serum sodium concentration.
Leave blank to use default TBW fraction (0.6 male, 0.5 female). Consider 0.45 in elderly or edematous patients.
Total body water
42.0 L
Sodium deficit
420.0 mEq
Equivalent 3% saline volume*
819 mL
*Assumes 3% NaCl contains 513 mEq/L; adjust for other fluids.
How to Use This Calculator
Define target sodium increase
Choose a conservative target (often +4 to +6 mEq/L in 24 hours) based on hyponatraemia chronicity and risk factors.
Enter patient weight and sex
The calculator estimates total body water using default fractions (0.6 for men, 0.5 for women). Override with a custom fraction if clinical scenario warrants.
Plan sodium replacement
Use the sodium deficit to approximate hypertonic or isotonic saline requirements. Divide replacement over time and monitor sodium frequently.
Formula
Total body water (TBW) = Body weight (kg) × TBW fraction
Typical TBW fraction: 0.6 (adult male), 0.5 (adult female), 0.45 (elderly or edematous)
Sodium deficit (mEq) = TBW × (Target Na⁺ − Current Na⁺)
Volume of 3% saline (mL) ≈ (Sodium deficit ÷ 513) × 1000
Full Description
Estimating sodium deficit helps tailor hypertonic saline therapy for hyponatraemia. The calculation assumes uniform sodium distribution across total body water. Adjust TBW based on age, sex, and body composition. Replace sodium gradually, reassessing serum sodium every few hours. Always integrate clinical status, volume, and ongoing losses when planning therapy.
Frequently Asked Questions
Does the deficit need full replacement?
Not always. Often only part of the deficit is replaced initially (4–6 mEq/L rise), reserving further correction for later.
How accurate is the TBW estimate?
TBW fractions are approximations. Adjust downward (0.45) for elderly, heart failure, or cirrhosis; upward (0.65) for lean young men.
Can this be used for hypernatremia?
The same concept applies, but calculate free water deficit instead. Use the total body water calculator or hypernatremia-specific tools.
How frequently should sodium be monitored?
During active correction, check every 2–4 hours. Stop or reverse therapy if sodium rises too quickly.