Sodium Deficit Calculator
Sodium Deficit Calculator
Calculate the sodium deficit in hyponatraemia between a measured and target sodium, with correction rate limits.
Sodium Deficit
Sodium deficitWeight 70 kg, sodium 118 mmol/L, target 126 mmol/L, adult male total body water fraction 0.6
Formula
- TBW
- total body water in litres — weight in kg × the fraction for age and sex
- target Na
- the sodium you are aiming for, chosen to respect the 24-hour correction limit — not necessarily 140
- result
- mmol of sodium required; divide by the infusate's concentration to get a volume
Worked example
Weight 70 kg, sodium 118 mmol/L, target 126 mmol/L, adult male total body water fraction 0.6
TBW = 70 × 0.6 = 42 L
Target − measured = 126 − 118 = 8 mmol/L
42 × 8 = 336 mmol Na⁺
24-hour correction limits
| Risk factors present | Maximum correction |
|---|---|
| Chronic hyponatraemia, alcohol use disorder, malnutrition, liver disease, hypokalaemia | ≤ 8 mmol/L per 24 hours |
| None of the above | 10 – 12 mmol/L per 24 hours |
The correction limit is the safety point, not the deficit
The calculated deficit tells you how much sodium is needed to reach a chosen target; the correction limit tells you how quickly that target may safely be approached, and it is the more important number to get right. No more than 8 mmol/L in any 24-hour period in patients at risk of osmotic demyelination syndrome — chronic hyponatraemia, alcohol use disorder, malnutrition, liver disease or hypokalaemia — and 10–12 mmol/L in others. Overshooting is common because ongoing free water diuresis, once the stimulus to ADH is removed, can raise sodium far faster than an infusion alone would predict.
In severe symptomatic hyponatraemia — seizures, coma, or other severe neurological signs — the initial treatment is not a calculated infusion at all. It is a 100–150 mL bolus of 3% saline, aiming to raise sodium by 4–6 mmol/L quickly to relieve cerebral oedema, then stopping to reassess. The deficit calculation becomes relevant afterwards, for planning the more gradual correction of the remaining gap to the target sodium.
Knowing the concentration of common infusates turns the calculated deficit into a volume: 3% saline contains 513 mmol/L of sodium, and 0.9% saline contains 154 mmol/L. Whichever fluid is chosen, sodium should be rechecked every few hours during active correction — the calculation is a starting estimate, not a substitute for measurement.
Frequently asked questions
Why is the correction limit lower in some patients?
Patients with chronic hyponatraemia, alcohol use disorder, malnutrition, liver disease or hypokalaemia are at higher risk of osmotic demyelination syndrome, so their correction is capped at 8 mmol/L per 24 hours rather than 10–12.
What is the initial treatment for severe symptomatic hyponatraemia?
A 100–150 mL bolus of 3% saline, aiming to raise sodium by 4–6 mmol/L quickly, then stopping to reassess — not a calculated infusion run over hours.
How much sodium is in 3% saline compared with 0.9% saline?
3% saline contains 513 mmol/L of sodium; 0.9% saline contains 154 mmol/L. Dividing the calculated deficit by these values gives the required infusate volume.
Does this calculator replace frequent sodium monitoring?
No. It gives a starting estimate. Sodium should be rechecked every few hours during active correction because ongoing losses and free water diuresis can change the rate of rise unpredictably.
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References
- Sterns RH. Disorders of plasma sodium — causes, consequences, and correction. N Engl J Med. 2015;372(1):55–65.
- Verbalis JG et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1–42.