Sodium Deficit Calculator
Find the sodium deficit in hyponatremia.
Result
How it is calculated
The tool first estimates total body water, then multiplies it by the sodium gap: TBW = 0.6 × weight and deficit = TBW × (target − current). Weight is in kilograms, current and target serum sodium are in mmol/L (numerically the same as mEq/L for sodium), so TBW comes out in litres and the deficit in mmol. The factor 0.6 is fixed; the tool does not switch to a lower fraction for women, older adults or people with obesity, so in those patients the true water volume is smaller than shown. All three inputs must be greater than zero. If the target is below the current value the deficit is negative, which is not a replacement amount. The figure is a textbook estimate, not a prescription or an infusion rate.
Worked example
Take a 70 kg patient with a serum sodium of 120 mmol/L and a target of 130 mmol/L. Total body water is 0.6 × 70 = 42.00 L. The gap is 130 − 120 = 10 mmol/L. The deficit is 42 × 10 = 420.00 mmol. The calculation says nothing about how fast to correct; safe daily correction limits must be applied separately by the treating clinician. Swapping the two sodium values gives -420.00 mmol, which signals the entries are reversed.
Common mistakes
- Swapping current and target sodium, which turns the deficit negative.
- Entering weight in pounds instead of kilograms; TBW and the deficit are then about 2.2 times too high.
- Assuming the tool adjusts TBW for sex or age; it always uses 0.6 × weight, so the volume is overstated for women and the elderly.
- Choosing a target far above the current value and treating the full deficit as the amount to give at once; the number does not include any limit on how quickly sodium may rise.
- Leaving a field at 0 (for example a blank lab value); the tool then returns no result.
About this calculator
In hyponatremia, the sodium deficit estimates how much sodium (in mEq) would need to be replaced to raise a person's serum sodium from its current, low level to a chosen target: Sodium deficit = TBW × (Target Na − Current Na), where total body water (TBW) is approximated as body weight in kilograms times a fraction — roughly 0.6 in men and 0.5 in women, lower in the elderly — reflecting that sodium distributes through all of the body's water, not just the bloodstream.
This calculation, closely related to the Adrogué-Madias approach to correcting sodium disorders, is a bedside tool in hospital medicine and nephrology: it gives clinicians a starting estimate of the correction needed, which is then weighed against strict safe correction-rate limits to avoid complications like osmotic demyelination from correcting sodium too quickly.
Physicians, nurses, and medical students working through electrolyte disorders use it as a teaching and quick-reference calculation — it estimates the deficit only, not a treatment plan, since actual correction always accounts for the rate limits and clinical context an individual patient requires.
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