Correcting sodium too fast is dangerous: rising too quickly in chronic hyponatraemia can cause osmotic demyelination, and falling too fast in hypernatraemia can cause brain swelling. The formula gives a starting estimate; sodium must be rechecked every few hours.
How to use the Sodium Change Calculator
- Weight
- Patient group (total body water)
- Serum sodium
- Infusate
- Potassium added to infusate
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter weight, patient group, current sodium and the fluid you plan to give.
Inputs explained
| Field | What to enter |
|---|---|
| Weight | Units: kg, lb, st, g |
| Patient group (total body water) | Choose one: Adult man (60% of weight), Adult woman (50%), Older man (50%), Older woman (45%) … |
| Serum sodium | In mEq/L |
| Infusate | Choose one: 3% saline (513 mEq/L), 0.9% saline (154 mEq/L), Lactated Ringer’s (130 mEq/L), 0.45% saline (77 mEq/L) … |
| Potassium added to infusate | In mEq/L |
What each result means
- Total body water = [Weight]×[Patient group (total body water)]
- Change in sodium per litre = ([Infusate] + [Potassium added to infusate] – [Serum sodium])÷([Total body water] + 1)
- Litres to change sodium by 8 mEq/L = abs(8÷[Change in sodium per litre])
Example values
How the main result changes when Weight changes and everything else stays at the example values above:
| Weight (kg) | Change in sodium per litre (mEq/L) |
|---|---|
| 35 | 17.9 |
| 52.5 | 12.1 |
| 70 | 9.1 |
| 87.5 | 7.3 |
| 105 | 6.1 |
| 140 | 4.6 |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
TBW = weight × factor (0.6 men, 0.5 women, 0.5 older men, 0.45 older women, 0.6 children)
ΔNa per litre = (infusate Na + infusate K − serum Na) ÷ (TBW + 1)
Worked example
Inputs — Weight: 70 kg; Patient group (total body water): Adult man (60% of weight); Serum sodium: 120 mEq/L; Infusate: 3% saline (513 mEq/L); Potassium added to infusate: 0 mEq/L.
Results — Total body water = 42 L; Change in sodium per litre = 9.1 mEq/L; Litres to change sodium by 8 mEq/L = 0.88 L.
Key terms
- TBW
- Total body water.
- ODS
- Osmotic demyelination syndrome.
Frequently asked questions
What is a safe correction rate?
In chronic hyponatraemia, no more than 8–10 mEq/L in 24 hours (8 in high-risk patients).
Why add potassium?
Potassium also raises sodium because it moves into cells in exchange.
Is the formula exact?
No – ongoing losses and urine output change sodium too; measure frequently.