Sodium Deficit Calculator

By Małgorzata Koperska, MD
Last updated: Jul 07, 2018

The calculation of sodium deficit is necessary when aiming to keep the patient in the state of optimal electrolyte balance.

The provided equation uses the patient's weight, their serum sodium (Na⁺) concentration, and the desired serum sodium concentration. Assignment to the correct age/sex group is also needed for the best estimation of total body water, which is included in the sodium deficit formula.

What is sodium deficit?

Sodium deficit is the quantity of sodium missing from the patient's organism in the state of hyponatremia, expressed in millimoles (mmol).

  • Hyponatremia is a low sodium level in the blood serum, most commonly described as a sodium concentration below 135 mEq/L (or mmol/L). The first symptoms are usually headaches, confusion, nausea, and imbalance.
  • Severe hyponatremia is a state where the patient's sodium level is below 125 mEq/L (or mmol/L). The symptoms of the sodium level falling down are dry mucus membranes, cold and clammy skin, tremors, and seizures.

Remember that hyponatremia should be corrected slowly, as in case of overcorrection there is a risk of osmotic demyelination. The optimal sodium correction rate is 0,5 mEq/L per hour. You can use our calculator for hyponatremia correction rate to estimate the flow of fluid you're planning to use.

During the fluid supplementation, electrolytes in the patient's blood serum should be tested frequently. Note that in the state of hyperglycemia, the sodium measurements can be distorted. Use the sodium correction calculator if your patient's blood sugar is high.

What's the sodium deficit formula?

The sodium deficit is calculated using this equation: Sodium deficit = TBW * ( desired_sodium - serum_ sodium) where:

  • TBW - total body water, calculated by multiplying the patient's weight by the age/sex coordinate, which is 60% for children and adult males, 50% for adult females and elderly males, and 45% for elderly females.
  • desired_sodium - the aimed sodium concentration, usually it's 140 mEq/L.
  • serum_sodium - the measured level of sodium concentration in the patient's blood serum.

The result is expressed in millimoles (mmol).

Małgorzata Koperska, MD
Age/sex category
adult female
Serum sodium
Desired sodium
Sodium deficit
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