Urea crosses cell membranes freely, so it does not move water; effective osmolality (tonicity) is what matters for brain cells in hyponatraemia and hyperglycaemic states.
How to use the Plasma Osmolality Calculator
- Sodium
- Potassium
- Glucose
- BUN
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the electrolytes, glucose and BUN or urea.
Inputs explained
| Field | What to enter |
|---|---|
| Sodium | In mmol/L |
| Potassium | In mmol/L |
| Glucose | Units: mg/dL, mmol/L |
| BUN | Units: mg/dL, mmol/L urea |
What each result means
- Plasma osmolality (2Na + glucose + BUN) = 2×[Sodium] + [Glucose]÷18 + [BUN]÷2.8
- With potassium (2(Na + K) + glucose + urea) = 2×([Sodium] + [Potassium]) + [Glucose]÷18 + [BUN]÷2.8
- Effective osmolality (tonicity) = 2×[Sodium] + [Glucose]÷18
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
Osmolality = 2 × (Na + K) + glucose + urea (all mmol/L)
US units: 2 × Na + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8
Effective osmolality (tonicity) = 2 × Na + glucose ÷ 18
Worked example
Inputs — Sodium: 140 mmol/L; Potassium: 4 mmol/L; Glucose: 100 mg/dL; BUN: 14 mg/dL.
Results — Plasma osmolality (2Na + glucose + BUN) = 291 mOsm/kg; With potassium (2(Na + K) + glucose + urea) = 299 mOsm/kg; Effective osmolality (tonicity) = 286 mOsm/kg.
Key terms
- Tonicity
- Effective osmolality.
Frequently asked questions
What is a normal plasma osmolality?
About 275–295 mOsm/kg.
What is effective osmolality?
Osmolality from substances that do not cross cell membranes freely – mainly sodium and glucose.
When is it important?
In hyperosmolar hyperglycaemic state, an effective osmolality over 320 mOsm/kg is a diagnostic criterion.