Sodium Deficit Calculator
Estimate the total sodium deficit (in mEq) used to plan correction of hyponatremia. The tool multiplies total body water by the gap between the target and current serum sodium, then shows a safe correction time and, optionally, the volume of replacement fluid that supplies that sodium. Results are an estimate; sodium correction must be supervised by a clinician.
Formula
Worked example
A 55 kg adult female (factor 0.5) with serum sodium 125 mEq/L and a target of 140 mEq/L: TBW = 55 × 0.5 = 27.5 L, gap = 140 - 125 = 15, deficit = 27.5 × 15 = about 413 mEq. With an 8 mEq/L daily ceiling, day one supplies 27.5 × 8 = 220 mEq.
What the sodium deficit tells you
The sodium deficit estimates the total amount of sodium (in milliequivalents) needed to raise a patient with hyponatremia from their current serum sodium to a target value. It is found by multiplying total body water by the difference between the target and current sodium concentrations. Total body water itself is estimated as a fraction of body weight: about 0.6 for children and adult males, 0.5 for adult females and elderly males, and 0.45 for elderly females, reflecting that body water declines with age and tends to be lower in females. The result is the size of the gap to close, not the rate at which to close it.
Correction rate, daily ceiling and a day-by-day plan
Closing the gap safely matters more than the total number. This tool turns the deficit into a time estimate using a cautious correction rate (commonly about 0.5 mEq/L per hour) and a 24 hour ceiling on how far serum sodium may rise (often 8 mEq/L, or 6 in high-risk patients). It also shows how much sodium the first 24 hours should supply, and a projected schedule that raises serum sodium by at most the daily ceiling each day until the target is reached. These figures are planning aids: the actual rate is titrated against repeat blood tests, because overshooting is the danger.
Optional replacement-fluid volume
Turn on the fluid estimate to convert the sodium deficit into a rough volume of intravenous fluid that would supply it. Select the fluid (3% hypertonic saline at 513 mEq/L, 0.9% normal saline at 154 mEq/L, or 0.45% half-normal saline at 77 mEq/L) and the tool divides the deficit by that concentration. This is a simplified figure that ignores ongoing sodium and water losses, urinary output, and the dilution effect of the water in the fluid itself, so the true volume and rate are always set by the treating team.
Why correction must be slow and supervised
Hyponatremia should usually be corrected gradually. Raising serum sodium too quickly can trigger osmotic demyelination syndrome, a serious and sometimes irreversible brain injury. A widely cited ceiling is a rise of no more than about 8 mEq/L in any 24 hour period, and even slower in patients at high risk such as those with chronic hyponatremia, malnutrition, alcohol use disorder, or liver disease. Because of these risks, the deficit figure is only a planning estimate: the choice of fluid, the infusion rate, and the frequency of repeat sodium measurements are clinical decisions that require a qualified professional and close monitoring.
Total body water (TBW) factor by group
| Group | TBW factor | TBW at 70 kg |
|---|---|---|
| Child | 0.60 | 42.0 L |
| Adult male | 0.60 | 42.0 L |
| Adult female | 0.50 | 35.0 L |
| Elderly male | 0.50 | 35.0 L |
| Elderly female | 0.45 | 31.5 L |
TBW is estimated as a fraction of body weight. Body water falls with age and is lower in females on average.
Frequently asked questions
Is the sodium deficit the same as the infusion rate?
No. The deficit is the total amount of sodium needed to reach the target. The rate of correction is separate and is deliberately limited (commonly to about 0.5 mEq/L per hour, capped at roughly 8 mEq/L per 24 hours) to avoid osmotic demyelination. A clinician converts the deficit into a fluid choice and infusion rate.
What total body water factor should I use?
Common factors are 0.6 of body weight for children and adult males, 0.5 for adult females and elderly males, and 0.45 for elderly females. These reflect that body water falls with age and is generally lower in females. They are population estimates and individual patients vary.
How long should it take to correct the sodium?
The tool divides the sodium gap by your chosen safe rate and also enforces the 24 hour ceiling, then reports the longer of the two as the minimum time. For a 15 mEq/L gap at 0.5 mEq/L per hour with an 8 mEq/L daily limit, correction is spread over about two days. Always titrate to repeat blood tests rather than a fixed clock.
How much fluid does the deficit represent?
Switch on the fluid estimate and pick a saline concentration. The deficit divided by the fluid sodium gives a volume: for example a 413 mEq deficit is about 805 mL of 3% saline (513 mEq/L). This ignores ongoing losses and the water the fluid adds, so treat it as a rough upper bound, not a prescription.
Can I use this to treat hyponatremia myself?
No. This is an educational estimate only. Correcting sodium too fast can cause permanent brain injury, so treatment must be directed by a qualified clinician with frequent blood tests. Clinical decisions should never rely on a calculator alone.