Estimate plasma osmolality from sodium, glucose, and BUN (or urea). If you have a directly measured serum or plasma osmolality, enter it to calculate a gap. Use results from the same blood draw when possible.
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Formula and example
Estimated osmolality ≈ 2 × Na (mmol/L) + glucose (mmol/L) + urea (mmol/L). Glucose in mg/dL is divided by 18; BUN (urea nitrogen) in mg/dL by 2.8; a separately reported urea value in mg/dL by 6. Effective osmolality (tonicity) omits urea: 2 × Na + glucose.
For Na 140 mmol/L, glucose 90 mg/dL and BUN 14 mg/dL, estimated osmolality is 280 + 5 + 5 = 290 mOsm/kg, and effective osmolality is 285 mOsm/kg. A directly measured value of 295 mOsm/kg yields a gap of 5 mOsm/kg.
Estimated, measured and effective osmolality
The calculation does not replace direct laboratory measurement. Urea passes relatively freely through cell membranes, so it is excluded from effective osmolality. Interpret results using the reporting laboratory’s reference interval and the clinical picture.
Osmolal gap = directly measured osmolality − the estimate on this page. A difference of about 10 mOsm/kg or more can be a clue to unmeasured solutes but is not specific. Ethanol, mannitol, formula choice and laboratory method affect it. A small gap cannot rule out toxic alcohol exposure; seek urgent care if exposure is suspected.
What to do next
Distinguish “BUN” from “urea” on the report and confirm the units. Review sodium, glucose, kidney results, measured osmolality, medicines and possible exposures with a clinician.
Frequently asked questions
Are BUN and urea the same number?
No. BUN reports the nitrogen portion of urea. Select the exact test name and unit printed on your report.
Can I calculate without a measured osmolality?
Yes. Sodium, glucose and BUN or urea give an estimate and effective osmolality. No osmolal gap appears unless a measured value is entered.
Does a small gap exclude poisoning?
No. A small gap cannot rule out toxic alcohol exposure. If ingestion is suspected or severe symptoms occur, seek urgent medical evaluation regardless of this result.
Related tools and records
Anion gap calculator · Urine ACR calculator · Health records · All calculators
Sources
Merck Manual Professional: Water and Sodium Balance · Gloucestershire Hospitals NHS: Osmolality · Hoffman et al.: limitations of the osmolal gap
Sources reviewed September 30, 2026. General information, not a diagnosis or substitute for clinical care. No independent clinician review is claimed.