π§ Sodium Deficit Calculator
How much sodium is missing in hyponatremia?
Estimated sodium deficit
For informational purposes only, not medical advice.
Sodium deficit equals total body water multiplied by the gap between target and current sodium. For a 70 kg man with sodium of 120 aiming for 130, total body water is about 42 liters, and the deficit is 42 times 10, or 420 mEq of sodium. The formula turns two lab numbers and a weight into a concrete replacement figure.
Total body water is estimated at 60 percent of weight for men and 50 percent for women, the standard bedside fractions. This is an educational estimate: real replacement math also weighs ongoing losses, kidney function, and the cause, which is why the number informs the clinician rather than writing the order.
How to Use This Calculator
Enter weight in kilograms, select sex for the body-water fraction, then enter the current sodium and the target. The tool multiplies it out and shows the water volume and the deficit side by side.
Enter the patient's weight in kilograms. Divide pounds by 2.2 if needed.
Select sex. The 0.6 versus 0.5 fraction reflects average body composition differences.
Enter current and target sodium. Keep the target modest: most corrections aim for a rise of 4 to 6 mEq/L per day, not full normalization at once.
The Formula
The logic is dilution in reverse. Sodium distributes through total body water, so each liter of that water needs the concentration raised by the gap. Multiply the gap by the liters and you get the milliequivalents missing. It is the same idea as figuring out how much salt seasons a pot: volume times the concentration shortfall.
The body-water fractions are averages and they drift with age and body composition. Elderly and obese patients carry less water per kilogram, muscular patients more. Treat the 0.6 and 0.5 as starting estimates, and expect the clinician to adjust for the person in front of them.
Worked Example
A 70 kg man has a sodium of 120 mEq/L and the team targets 130. Total body water is 0.6 times 70, which is 42 liters. The gap is 10 mEq/L. Multiply 42 by 10 to get a deficit of 420 mEq of sodium.
What does 420 mEq mean at the bedside? Three percent hypertonic saline carries about 513 mEq per liter, so the deficit equals roughly 0.8 liters of 3 percent saline, before accounting for ongoing losses. Nobody gives the whole deficit at once; the number frames the scale of replacement while the correction-rate ceiling governs the speed.
Reading Your Result
A few hundred milliequivalents is a typical moderate deficit; over a thousand suggests severe or long-standing hyponatremia. But the deficit alone never sets the plan. Two patients with identical deficits need very different care if one is volume-depleted and the other is overloaded with heart failure.
| Context | What it means for the number |
|---|---|
| Hypovolemic hyponatremia | Isotonic fluids often fix both volume and sodium; the deficit guides how much. |
| Euvolemic (SIADH) | Water restriction or hypertonic saline; giving isotonic fluid can worsen it. |
| Hypervolemic (heart failure, cirrhosis) | Sodium is low because water is high; the answer is water removal, not salt loading. |
My take: the deficit formula is most useful as a reality check. If the ordered saline would deliver triple the deficit, something is off. If it delivers a tenth, the team is likely staging the correction across days, which is usually deliberate.
Limits
The formula assumes sodium distributes evenly through total body water and ignores intracellular shifts, ongoing urinary losses, and the water content of the infusions themselves. It also assumes the current sodium is accurate and not diluted by high glucose; run the sodium correction calculator first when sugar is high.
Most the deficit does not tell you which fluid to use or how fast to give it. Those decisions hinge on volume status and symptoms. This page answers how much is missing, and stops there on purpose.
Related Tools
Correct for glucose first with the sodium correction calculator, then check the pace with the safe sodium correction rate calculator. For everyday balance, the daily water intake calculator and blood pressure checker keep sodium in its daily context.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is the sodium deficit formula?
Sodium deficit in mEq equals total body water in liters times the difference between target and current sodium. Total body water is estimated as 0.6 times weight in kg for men and 0.5 times weight in kg for women. A 70 kg man going from 120 to 130 has a deficit of about 420 mEq.
Why does the formula use 0.6 for men and 0.5 for women?
Those fractions estimate total body water as a share of weight. Women on average carry more adipose tissue, which holds little water, so the fraction is lower. Both are population averages; age, obesity, and muscle mass shift the true value for any individual.
How much 3% saline does the deficit equal?
Three percent saline contains about 513 mEq of sodium per liter. Divide the deficit by 513 to get the equivalent volume. A 420 mEq deficit is roughly 0.8 liters of 3% saline, though clinicians rarely replace the full deficit at once and always respect the correction-rate ceiling.
Should the full deficit be replaced?
Usually not in one go. The deficit describes the total shortfall, but correction proceeds at a safe pace, often 4 to 6 mEq/L per day. Replacing everything immediately would violate the 8 mEq/L per 24-hour ceiling and risk osmotic demyelination.
Does the deficit tell me which IV fluid to use?
No. Fluid choice depends on volume status: hypovolemic patients often get isotonic saline, euvolemic SIADH patients may need hypertonic saline or water restriction, and overloaded patients need water removal. The deficit is a quantity; the fluid is a separate clinical decision.
What if glucose is high?
Correct the sodium for hyperglycemia first, because the measured value is diluted. Use the corrected value as the current sodium in this formula. Otherwise you will calculate a deficit that does not really exist and overtreat.
Is the formula accurate in kidney failure?
Less so. Oliguric patients cannot excrete the water or handle the sodium load normally, and dialysis changes the math entirely. The bedside formula is a starting estimate; nephrology input guides actual management in renal failure.
Can I use pounds instead of kilograms?
Convert first by dividing pounds by 2.2. The body-water fractions are defined per kilogram, so entering pounds directly would inflate the deficit by more than double. A 154 lb person is 70 kg.
What is a normal sodium deficit?
There is no normal deficit; healthy people have none because their sodium sits in range. The concept only applies once hyponatremia is diagnosed. Larger deficits generally track with more severe or longer-standing low sodium.
Who should not act on this number alone?
Anyone without clinical training managing real hyponatremia. The deficit is one input among many, including symptoms, volume exam, kidney function, and cause. Bring the number to the medical team as a discussion point, not an order.