Sodium and its anions are the main contributors to osmolality, with glucose and urea adding smaller amounts.
How to use the Serum Osmolality Calculator
- Sodium
- Glucose
- BUN
- Measured osmolality (optional)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Double the sodium, add glucose divided by 18 and BUN divided by 2.8.
Inputs explained
| Field | What to enter |
|---|---|
| Sodium | In mEq/L |
| Glucose | In mg/dL |
| BUN | In mg/dL |
| Measured osmolality (optional) | In mOsm/kg |
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
Calculated Osm = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 (glucose and BUN in mg/dL)Worked example
Inputs — Sodium: 140 mEq/L; Glucose: 100 mg/dL; BUN: 14 mg/dL; Measured osmolality (optional): 295 mOsm/kg.
Results — Calculated osmolality = 291 mOsm/kg; Osmolal gap = 4 mOsm/kg.
Frequently asked questions
What is a normal value?
About 275–295 mOsm/kg.
What does a high osmolal gap suggest?
Unmeasured substances such as alcohols may be present.
What formula does the Serum Osmolality Calculator use?
Calculated Osm = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 (glucose and BUN in mg/dL).