Serum Osmolality & Osmolar Gap Calculator
Work out the calculated serum osmolality from sodium, glucose and urea, and — if you have a lab-measured osmolality — the osmolar gap between them. A raised osmolar gap points to unmeasured osmoles in the blood, classically the toxic alcohols such as methanol and ethylene glycol. Enter values in conventional US units; the ethanol and measured-osmolality fields are optional.
About the osmolar gap
The osmolar gap compares the osmolality the routine solutes predict with what the lab actually measures; a large gap flags unmeasured osmoles. It is used with the anion gap and clinical context in suspected toxic-alcohol poisoning. Formula per standard clinical references; a normal gap does not exclude poisoning.
Frequently asked questions
What is the calculated osmolality formula?
The common formula is: calculated osmolality = 2 × sodium + glucose ÷ 18 + BUN ÷ 2.8, with sodium in mmol/L and glucose and blood urea nitrogen (BUN) in mg/dL. If a serum ethanol level is known it is added as ethanol ÷ 3.7. Sodium is doubled to account for its accompanying anions; the divisors convert glucose, urea and ethanol from mg/dL to mmol/L.
What is the osmolar gap?
The osmolar gap is the measured (lab) osmolality minus the calculated osmolality. It represents osmotically active substances the formula does not include. A normal gap is generally under about 10 mOsm/kg. A raised gap suggests unmeasured osmoles — most importantly the toxic alcohols methanol and ethylene glycol, but also mannitol, and it can be falsely elevated by lab methods or severe hyperlipidaemia/hyperproteinaemia.
When is a raised osmolar gap important?
In a patient with a metabolic acidosis and a high anion gap, a raised osmolar gap raises concern for toxic-alcohol poisoning and can prompt urgent treatment before confirmatory levels return. A normal gap does not fully exclude it, especially late after ingestion when the parent alcohol has been metabolised, so the gap is used alongside the clinical picture and the anion gap. The two gaps move in opposite directions over the course of an ingestion: the osmolar gap is highest early, while the parent alcohol is still circulating, and the anion gap rises later as it is converted to its acid metabolites. That is why a normal osmolar gap in a late presentation is reassuring only when the anion gap and acid-base picture are normal too.
Do I need to enter ethanol?
Only if a serum ethanol level is available and you want it accounted for. Including known ethanol stops it from inflating the osmolar gap, so the remaining gap better reflects other unmeasured osmoles. Leave the field blank or zero if you do not have a level.
What units does this use?
Conventional US units: sodium in mmol/L, glucose, BUN and ethanol in mg/dL, and measured osmolality in mOsm/kg. To convert glucose from mmol/L multiply by 18; to convert urea (mmol/L) to BUN (mg/dL) multiply by 2.8. This calculator is an educational reference, not medical advice.
This is a reference tool, not a diagnosis or medical advice. Results depend on the laboratory, assay and clinical context, and reference intervals vary between labs and by age and sex — always read your result against the range printed on your own report and discuss it with a qualified healthcare professional.
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