Sodium Deficit Calculator
The sodium deficit is an estimate of how much sodium, in millimoles, must be added to raise a patient's serum sodium from its current level toward a target. It is one input into the planning of correction for hyponatremia. Over-rapid correction is dangerous, so the rate, not just the total, is critical and must be managed by a clinician. This tool computes the deficit from body weight, the total body water fraction, and current and target sodium. It is arithmetic only and not a treatment plan.
Sodium deficit formula
Sodium deficit (mmol) = total body water fraction * weight_kg * (target Na - current Na)
Weight is in kilograms and sodium in mmol/L. The total body water fraction is commonly taken as about 0.6 in men and children and 0.5 in women, with lower values in the elderly; it is an editable input so you can match your reference. The result is the millimoles of sodium needed to reach the target if other factors are held constant.
Worked example
For a 70 kg patient with total body water fraction 0.6, current sodium 120, and target 130 mmol/L: deficit = 0.6 * 70 * (130 - 120) = 0.6 * 70 * 10 = 420 mmol. This is the estimated amount to raise sodium by 10 mmol/L, to be given at a clinician-controlled rate.
Frequently asked questions
What total body water fraction should I use?
Common reference values are about 0.6 for non-elderly men and children, 0.5 for non-elderly women, and lower (around 0.45 to 0.5) for the elderly. Because sources differ, this is a user-editable input rather than a hardcoded constant.
Does the deficit tell me how fast to correct sodium?
No. The deficit is a total amount, not a rate. Correcting hyponatremia too quickly can cause osmotic demyelination, so guidelines limit the rise per 24 hours. Rate planning must be done by a clinician.
Is this the Adrogue-Madias formula?
It is the related sodium deficit equation. The Adrogue-Madias formula predicts the change in serum sodium from a given infusate and is a different calculation; this tool computes the total deficit to a target.
Can this replace clinical management?
No. It performs one arithmetic step. Treatment of hyponatremia requires diagnosis of the cause, choice of fluid, rate limits, and monitoring, all of which are clinical decisions.
Sources
- U.S. National Library of Medicine, StatPearls: Hyponatremia.
- U.S. National Library of Medicine, MedlinePlus: Sodium in Diet and Blood.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. Educational tool, not medical advice. See our methodology.