Sodium Deficit Calculator

Estimate total body water from weight, sex, and age, then calculate the sodium (mEq) needed to move serum sodium from its current level to your target, plus a minimum safe correction time.

Quick Facts

Formula
Deficit (mEq) = TBW x (target Na - current Na)
TBW (L) = weight (kg) x fraction (0.6/0.5 non-elderly male/female, 0.5/0.45 elderly male/female).
Normal serum sodium
135-145 mEq/L
Values outside this band define hyponatremia (low) or hypernatremia (high).
Common correction limit
~8 mEq/L per 24 hours
A frequently cited ceiling to reduce osmotic demyelination risk; clinicians may use a different limit.

Your Results

Calculated
Total body water
-
Estimated TBW (liters)
Sodium deficit
-
mEq needed to reach target
Sodium change needed
-
Target minus current (mEq/L)
Min. correction time
-
At the entered max rate

Ready

Enter sex, age, weight, and the current and target sodium levels, then calculate.

About the sodium deficit calculation

The sodium deficit calculation estimates how many milliequivalents (mEq) of sodium are needed to move a patient's serum sodium from its current, measured level to a chosen target level. It is built on a two-step model: first estimate total body water (TBW), then multiply that volume by the desired change in sodium concentration.

Step 1: estimate total body water

TBW is approximated as body weight in kilograms multiplied by a fraction of body weight that is water. The fraction commonly used in clinical calculators is 0.6 for non-elderly men, 0.5 for non-elderly women or elderly men, and 0.45 for elderly women, where "elderly" is defined as age 65 or older. This is a population-average simplification, not a direct body-composition measurement.

Step 2: calculate the deficit

Sodium deficit (mEq) = TBW (L) x (target serum sodium - current serum sodium), both expressed in mEq/L. A positive result means sodium needs to be added to raise a low sodium level (hyponatremia); a negative result means the current level is above the target (hypernatremia), where the correction direction is instead to lower sodium, typically with free water.

Why the correction rate matters

Serum sodium should not be corrected too quickly. Raising sodium faster than roughly 8 mEq/L in 24 hours is a commonly cited threshold associated with a higher risk of osmotic demyelination syndrome, a serious neurological injury. This calculator divides the total sodium change you have set by the maximum rate you specify to estimate the minimum number of hours the correction should take.

When to consult a professional

This tool performs the standard TBW and sodium deficit arithmetic for education and planning. It does not choose a fluid, set an infusion rate, or replace clinical judgment. Correcting hyponatremia or hypernatremia should be planned and monitored by a treating clinician, with frequent sodium rechecks.

Frequently Asked Questions

What formula does the sodium deficit calculator use?
It uses the standard clinical formula: total body water (TBW) in liters equals body weight in kilograms times a TBW fraction based on sex and age (0.6 for non-elderly men, 0.5 for non-elderly women or elderly men, and 0.45 for elderly women, elderly meaning 65 or older). The sodium deficit in mEq then equals TBW times the difference between the target and current serum sodium.
How is the minimum correction time calculated?
The calculator divides the absolute change in serum sodium (target minus current, in mEq/L) by the maximum correction rate you enter, then multiplies by 24 hours. This mirrors the common clinical guideline of limiting serum sodium change to about 8 mEq/L per 24 hours to reduce the risk of osmotic demyelination syndrome.
Does this tool replace a clinician's dosing decision?
No. This calculator performs the standard TBW and sodium deficit arithmetic for education and planning. Fluid choice, infusion rate, and monitoring frequency for correcting hyponatremia or hypernatremia should be determined and supervised by a treating clinician.

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