Calculate serum osmolality from sodium, glucose and urea (BUN) in mg/dL or mmol/L, the effective osmolality, and the osmolal gap against a measured value, with an optional ethanol term. For healthcare professionals.
Effective osmolality leaves out urea, which cross cell membranes: 280 + 5 = 285.
For healthcare professionals. A calculation from a standard formula, not an interpretation: verify against current guidelines and your laboratory.
How to use the serum osmolality calculator
Choose mg/dL (glucose and BUN, as in the US) or mmol/L (glucose and urea, as in the UK, Europe and India). Values typed so far are converted.
Enter sodium, glucose and BUN or urea from the same sample, and ethanol if it was measured.
Add the measured osmolality from the laboratory to see the osmolal gap.
Worked example
Sodium 140 mmol/L, glucose 90 mg/dL and BUN 14 mg/dL give 2 × 140 + 90 ÷ 18 + 14 ÷ 2.8 = 280 + 5 + 5 = 290 mOsm/kg. If the laboratory measures 320 mOsm/kg, the osmolal gap is 320 − 290 = 30. An ethanol level of 100 mg/dL would explain 100 ÷ 3.7 = 27 of it, leaving a gap of about 3.
Units
Glucose: mg/dL ÷ 18 = mmol/L (18.016 exactly; the formula uses 18).
BUN to urea: BUN mg/dL ÷ 2.8 = urea mmol/L. Urea in mg/dL (used in India) is BUN × 2.14.
Ethanol: mg/dL ÷ 4.607 = mmol/L.
Sources: Dorwart WV, Chalmers L, Clin Chem 1975;21:190 (formula); Purssell RA et al., Ann Emerg Med 2001;38:653 (ethanol factor). For healthcare professionals: a calculation, not an interpretation.
Frequently asked questions
What is the formula for calculated serum osmolality?
With glucose and blood urea nitrogen in mg/dL: 2 × Na⁺ + glucose ÷ 18 + BUN ÷ 2.8. With glucose and urea in mmol/L the divisors disappear: 2 × Na⁺ + glucose + urea. Sodium is doubled to count its accompanying anions. For sodium 140, glucose 90 mg/dL and BUN 14 mg/dL the result is 280 + 5 + 5 = 290 mOsm/kg.
What is the osmolal gap?
The measured osmolality from the laboratory minus the calculated value. A gap means something osmotically active is in the blood that the formula doesn't count, such as ethanol, methanol, ethylene glycol or mannitol. Laboratories and formulas differ, so interpret a gap against your own reference values, and remember a normal gap doesn't exclude a late toxic alcohol ingestion.
How is ethanol included?
Ethanol in mg/dL divided by 3.7 (Purssell and colleagues found 3.7 fitted measured values better than the molecular-weight figure of 4.6), or its value in mmol/L added as it is. Adding the ethanol term shows how much of a gap ethanol explains.
What is effective osmolality?
Tonicity: the part of osmolality that moves water across cell membranes. Urea and ethanol cross cell membranes freely, so effective osmolality is 2 × Na⁺ + glucose. It is the figure used when thinking about water shifts in conditions such as hyperglycaemia.
