Free water deficit calculator
Enter the serum sodium, the body weight and the patient group to estimate how much free water is missing in hypernatremia. The result is an estimate for information and teaching; how fast to replace it, and with what fluid, is a clinical decision.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose the patient group: it sets the share of body weight that is water.
- Enter the weight in kg or lb and the measured serum sodium.
- Keep the target at 140 mmol/L or enter another one.
- Read the free water deficit and the estimated total body water.
- Total body water (L) = weight (kg) × fraction (0.6 child or adult man, 0.5 adult woman or older man, 0.45 older woman)
- Free water deficit (L) = total body water × (Na ÷ target Na − 1)
Worked example
An adult man weighing 70 kg with sodium 160 mmol/L: total body water is 70 × 0.6 = 42 L. The deficit is 42 × (160 ÷ 140 − 1) = 42 × 0.143 = 6 L.
The same man entered as 154 lb: 154 × 0.4536 = 69.9 kg, total body water 41.9 L and a deficit of 5.99 L, shown as 6 L.
Limitations
- The formula estimates the pure water missing. It ignores losses that continue during treatment, which can be large.
- It assumes the extra sodium comes from water loss; in sodium gain (for example after hypertonic fluids) the situation differs.
- Recheck sodium regularly during treatment instead of relying on one calculation.
- For information and teaching only, not a treatment plan.
Frequently asked questions
What is the free water deficit formula?
Free water deficit = total body water × (serum sodium ÷ target sodium − 1). Total body water is body weight × a fraction: 0.6 for children and adult men, 0.5 for adult women and older men, 0.45 for older women. The usual target is 140 mmol/L.
How fast should the deficit be replaced?
Usually over 48 hours or more in hypernatremia that developed slowly, because lowering sodium too quickly can cause brain swelling. Guidance commonly limits the fall to about 10–12 mmol/L in 24 hours. The ongoing losses (urine, stool, fever, breathing) must be replaced on top.
Are mmol/L and mEq/L the same for sodium?
Yes. Sodium has a charge of one, so 1 mmol/L equals 1 mEq/L and you can enter either number as it is.
Who counts as older?
The lower water fractions are usually applied to people over 65, whose bodies contain less water and more fat. They are rough averages; very lean or obese people differ from them.