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Calculates the estimated serum osmolality using sodium, glucose, and blood urea nitrogen (BUN). Useful in evaluating electrolyte disorders, dehydration, toxic alcohol ingestion, and calculating the osmolar gap.
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
Calculated serum osmolality is an estimate derived from the principal osmotically active solutes: calculated osmolality = 2 x sodium + glucose/18 + BUN/2.8, with glucose and BUN in mg/dL. It approximates the laboratory-measured osmolality when these three solutes account for essentially all of the plasma osmolality. Sodium is the dominant contributor; glucose and urea contribute to a smaller but clinically relevant degree. The calculated value supports the evaluation of sodium disorders and hydration status. When compared with a simultaneously measured osmolality, it yields the osmolar gap (measured minus calculated osmolality), a key screen for unmeasured osmotically active substances such as ethanol, methanol, ethylene glycol, and isopropanol. Limitations matter: the formula reflects only sodium, glucose, and urea, so it does not capture ethanol or other unmeasured solutes; it assumes sodium in mmol/L and glucose and BUN in mg/dL; and its agreement with measured osmolality depends on the laboratory method used. When the clinical question involves toxic ingestion, unmeasured solutes, or monitoring the correction of hypernatremia, a measured osmolality is preferable to relying on the calculated estimate alone. This is an educational decision-support estimate. It does not by itself diagnose volume status, dysnatremia, or toxic exposure, and a widened osmolar gap always requires a measured osmolality for confirmation.
Specialty
Internal MedicineCategory
Internal MedicineDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-06
Formula
Keywords
Normal Range
275–295 mOsm/kg
| Classification | Range |
|---|---|
Low osmolality | <274.1 |
Normal osmolality | 275–295 |
High osmolality | ≥296 |
Critically elevated osmolality | ≥320 |
Estimates serum osmolality from sodium, glucose, and blood urea nitrogen, supporting evaluation of electrolyte disorders, hydration status, and toxic ingestions.
Calculated serum osmolality = 2 x sodium + glucose / 18 + BUN / 2.8, with sodium in mmol/L and glucose and BUN in mg/dL.
Serum sodium in mmol/L, multiplied by 2 as the dominant osmotic contributor.
Serum glucose in mg/dL, divided by 18 before being added.
Blood urea nitrogen in mg/dL, divided by 2.8 before being added.
Normal serum osmolality is 275–295 mOsm/kg. Values below ~275 mOsm/kg suggest dilutional hyponatremia. Values ≥296 mOsm/kg are high and ≥320 mOsm/kg are critically elevated, indicating hypernatremia, hyperglycemia, uremia, or ingestion of osmotically active substances.
Serum osmolality is a core component of evaluating sodium disorders, guides the interpretation of the osmolar gap for toxic alcohol exposure, and informs the rate of hypernatremia correction.
A 45-year-old man with poor intake and nausea has sodium 140 mmol/L, glucose 200 mg/dL, and BUN 14 mg/dL.
Inputs:
Expected Result: Calculated osmolality = 2(140) + 200/18 + 14/2.8 = 280 + 11.1 + 5 = 296.1 mOsm/kg — high, warranting a measured osmolality to assess the osmolar gap.
The osmolar gap is the difference between measured and calculated osmolality. An elevated gap (>10 mOsm/kg) suggests unmeasured osmotically active substances such as methanol, ethylene glycol, or ethanol.
Ethanol contributes to measured osmolality but not to this calculated value. Add approximately ethanol (mg/dL)/4.6 to account for its contribution when estimating effective osmolality.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Confirm critical results with a measured osmolality and interpret within the clinical context.