Educational Serum Sodium Deficit Estimate

This page demonstrates a simplified total-body-water model sometimes used to discuss a low measured serum sodium value. It accepts sex, body weight, and serum sodium from 100 to less than 140 mmol/L. It is not a diagnostic or treatment calculator and must not be used for self-treatment or clinical orders.

Hyponatraemia has multiple causes and can be acute or chronic, symptomatic or asymptomatic, hypotonic or non-hypotonic, and associated with different volume states. Appropriate evaluation requires clinical history, examination, repeat laboratory testing, serum and urine studies, medications, and the rate of change—not this three-input equation.

Method and formula

The model uses a coefficient of male 0.6 or female 0.5 to estimate total body water (TBW):

• male selection: TBW = body weight × 0.6
• female selection: TBW = body weight × 0.5

It then uses a fixed target sodium of 140 mmol/L:

estimated sodium deficit = (140 − measured sodium) × estimated TBW

The coefficients are broad approximations and do not account for age, body composition, pregnancy, oedema, kidney function, ongoing water and electrolyte changes, glucose, or the cause and duration of the abnormal result.

Worked example

For the male selection, 70 kg, and measured sodium 130 mmol/L:

TBW = 70 × 0.6 = 42 L

(140 − 130) × 42 = 420 mmol

The output is a modelled difference to the fixed target. It is not the amount a person should ingest or receive intravenously.

How to interpret the result

Treat the number only as an illustration of how assumptions about distribution volume change an arithmetic estimate. Selecting the female coefficient for the same weight and sodium would produce 350 mmol because the assumed TBW is 35 L. Neither coefficient establishes an individual's true TBW.

The page does not calculate a saline concentration, volume, dose, or correction rate. Those decisions depend on symptoms, acuity, cause, fluid status, comorbidities, concurrent treatment, repeat sodium measurements, and clinician-directed limits intended to avoid neurological injury from overcorrection.

Clinical safety limits

• New confusion, seizure, loss of consciousness, severe headache, persistent vomiting, or other serious symptoms with a sodium abnormality requires urgent medical assessment.
• Do not consume salt, electrolyte products, or change fluid intake based on this result. Both low sodium and overly rapid correction can be dangerous.
• A laboratory value may require confirmation and interpretation alongside glucose, osmolality, kidney function, urine testing, and medications. The formula cannot determine the diagnosis.
• The accepted 100–139.9 mmol/L range is an interface boundary, not a definition of severity or a safe range for unsupervised use.

Sources

• European Society of Endocrinology et al., Clinical Practice Guideline on Diagnosis and Treatment of Hyponatraemia — diagnostic classification, evaluation, treatment context, and correction limits.
• NCBI Bookshelf, Hyponatremia — clinical causes, assessment, monitoring, and risks of correction.
• Sterns, Treatment Guidelines for Hyponatremia: Stay the Course — evidence and expert discussion of conservative correction limits and osmotic demyelination risk.

Editorial record

• Author: SoupCalc Editorial Team
• Method review: SoupCalc Clinical Safety Review
• Last reviewed: August 10, 2026
• Review scope: arithmetic model, target and coefficient disclosure, input boundaries, removal of concentrated-saline output, emergency warnings, and source alignment