The sodium deficit calculation is a standard first step in planning sodium replacement for hyponatremia: it converts a lab value gap (current sodium vs. target sodium) into a total quantity, in milliequivalents, using an estimate of the patient's total body water. This guide explains where the total-body-water factor comes from, why the deficit is a total amount rather than a rate, and the limits of this simplified approach.

What the sodium deficit represents

Serum sodium concentration reflects the balance between total body sodium and total body water. When sodium is low (hyponatremia), the goal of replacement therapy is usually to raise the concentration toward a safer or normal target. Because concentration is a ratio of sodium to water, closing that gap requires knowing not just the sodium difference but the volume of water it is dissolved in — the total body water (TBW).

The sodium deficit formula multiplies the sodium gap (target minus current, in mEq/L) by the estimated TBW (in litres) to get a total quantity in milliequivalents. This is the widely-used Adrogué–Madias framework for planning sodium and water replacement, and it is the standard starting point clinicians use before choosing a specific fluid, rate, and monitoring plan.

Why total body water is estimated by weight, sex, and age

Water makes up a different share of body weight depending on sex and age, mainly because lean tissue holds more water than fat tissue and lean mass differs by sex and declines with age. The calculator uses the standard bedside shorthand: 0.6 of body weight for adult men, 0.5 for adult women, and lower factors (0.5 for men, 0.45 for women) for patients 65 and older. This shorthand is a simplified approximation of the more detailed Watson regression formula, chosen for speed and ease of bedside use.

Because TBW scales the entire deficit, getting the factor right matters: using the adult factor for an elderly patient — or the male factor for a female patient — can meaningfully over- or under-state the amount of sodium that needs replacing.

A total amount, not a rate — and why the rate matters more

The number this calculator produces is the total sodium deficit needed to reach the target — it says nothing about how quickly that sodium should be given. In practice, the rate of correction is at least as clinically important as the total amount. Correcting chronic hyponatremia too quickly is a well-documented cause of osmotic demyelination syndrome, a serious and sometimes permanent neurological injury. Clinical guidelines generally cap the rate of sodium rise over 24- and 48-hour windows, and the specific fluid, infusion rate, and monitoring schedule are individualized decisions that go well beyond a single deficit number.

This calculator is a starting-point estimate for the total quantity involved — it does not compute or recommend an infusion rate, fluid type, or treatment schedule.

Limits of this estimate

The TBW-fraction shorthand is a population average, not a measurement of any individual's actual body water — it can be less accurate in obesity, malnutrition, edema, or unusual body composition. The deficit formula also assumes a single-compartment model and does not account for ongoing sodium or water losses, intravenous fluids already given, or the underlying cause of the sodium abnormality (which changes both the treatment approach and the safe correction rate). Use this calculator as an educational starting point, not as a substitute for individualized clinical assessment and monitoring by a qualified healthcare professional.