How this instrument works
This calculator answers a blunter question than a per-liter infusion formula does: total mass, not rate. It estimates total body water as a fraction of weight — the same age- and sex-adjusted fraction used across fluid-and-electrolyte arithmetic — then multiplies that volume by the gap between a current and a target reading. The result is a single number: how many milliequivalents of sodium, in total, would need to enter that estimated volume to move the level from where it sits to where it's meant to go.
It is a textbook mass-balance approach taught alongside, not instead of, the per-liter Adrogué-Madias method this site's separate sodium correction tool uses. That other calculator asks what one liter of a named fluid will do to today's reading; this one asks how much total sodium mass is missing, full stop, treating the estimated body-water volume as the container the deficit has to fill. Both lean on the same body-water estimate but answer different planning questions, and clinicians commonly use both together rather than picking one.
Because the fraction of weight assumed to be water is a population average, not a direct measurement, this total is a planning estimate rather than a fixed prescription — real fluid balance shifts as sodium and water are both given, so the number describes where things stand today, not a dose to deliver in one motion.
- Set Sex and Age (years) — together they set the total body water fraction used below.
- Enter Weight (kg), the body weight the water volume is scaled from.
- Enter Current serum sodium (mEq/L), today's reading.
- Enter Desired serum sodium (mEq/L), the target level to plan toward.
- Read the total sodium deficit in mEq — the mass estimated to close that gap.
Worked example — 60 kg woman, sodium 118 to 130
A 45-year-old woman weighs 60 kg and has a serum sodium of 118 mEq/L; the plan is to raise it toward 130 mEq/L. Her total body water fraction, as a woman under 65, is 0.5. The gap between target and current is 130 minus 118, or 12 mEq/L. Multiply 0.5 by 60 by 12 and the estimated deficit comes to 360 mEq of sodium.
An older patient scales differently: a 70-year-old man at 80 kg moving from 120 to 135 mEq/L uses the lower elderly-male fraction of 0.5 rather than 0.6, giving 0.5 × 80 × 15, or 600 mEq — nearly twice the first patient's total, driven by the larger weight and wider gap rather than by any difference in the arithmetic itself.
Questions
How is this different from the sodium correction (per-liter) calculator?
This tool totals mass — the mEq of sodium estimated to move a current reading to a target one, full stop. The per-liter tool instead predicts what one liter of a specific named fluid will do to today's reading, which depends on that fluid's own composition. Both draw on the same total-body-water estimate; a total deficit still has to be delivered as some combination of fluids, at which point the per-liter tool tells you what each one contributes.
Why does the target matter — shouldn't I always aim for a normal reading?
Correcting chronic low sodium too quickly risks osmotic demyelination syndrome, a serious neurological injury, so clinicians generally aim for a conservative intermediate target and a safe daily rate — commonly cited as roughly 8-10 mEq/L across 24 hours — rather than jumping straight to a textbook-normal number. Setting a modest target here reflects that safety limit, not a ceiling on what the body can eventually tolerate.
Does the deficit figure tell me how fast to give the sodium?
No — it is a total quantity, not a rate or a schedule. Translating a total deficit into an actual infusion plan means spreading it across a safe number of hours and rechecking the serum level along the way, since the body's response to sodium and fluid does not track this mass-balance estimate perfectly once treatment is under way.
Why does an elderly patient get a lower total body water fraction?
Total body water tracks lean tissue more than total weight, and lean mass declines with age while fat's share rises, so the standard estimate drops from 0.6 to 0.5 for men and 0.5 to 0.45 for women past age 65. It remains a population average rather than a measurement of any specific patient's actual composition, which is a known limit of this style of estimate.
Can I use this deficit number for someone with high, not low, sodium?
Not directly — the arithmetic assumes you are solving for a positive gap, target above current, to correct hyponatremia. Lowering an elevated sodium reading is a distinct clinical problem addressed by a different equation, the free water deficit approach, which estimates water to add rather than sodium mass, and it is worth using the calculator built for that direction instead.
References
- Adrogué HJ, Madias NE. Hyponatremia. N Engl J Med. 2000 (PubMed)
- Spasovski et al. Clinical practice guideline on hyponatraemia, 2014
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.