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Serum Osmolality Calculator
Serum osmolality can be measured in the laboratory or calculated from the main solutes that determine it — sodium, glucose, and urea. Comparing the two gives the osmolal gap: a large gap means unmeasured solutes are present, which is a classic clue in suspected toxic alcohol poisoning and a few other conditions.
Enter the values or tick the options above — the result updates automatically.
How to interpret the result
Measured serum osmolality is usually about 275–295 mOsm/kg. The osmolal gap (measured minus calculated) is normally about 10 or less. A clearly raised gap has a short classic list of causes — methanol, ethylene glycol, and other alcohols; mannitol; and states with heavy solute loads such as lactic acidosis or ketoacidosis. A normal gap does not fully exclude a toxic alcohol, especially late after ingestion when the parent alcohol has been metabolized (the anion gap is often high by then instead).
If your laboratory reports in SI units, convert first: glucose mg/dL = mmol/L × 18, and BUN mg/dL = urea mmol/L × 2.8.
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Frequently asked questions
What is the osmolal gap?
The difference between the laboratory-measured serum osmolality and the value calculated from sodium, glucose, and urea. A gap of about 10 mOsm/kg or less is typical; a larger gap means solutes are present that the formula does not account for.
Why is sodium doubled in the formula?
Because sodium travels with anions (mainly chloride and bicarbonate). Doubling the sodium concentration approximates the total contribution of sodium and its accompanying anions to osmolality.
Does ethanol count in calculated osmolality?
Yes — ethanol is osmotically active, so if it has been measured it should be included (ethanol in mg/dL divided by 4.6). Otherwise a person who has simply been drinking can appear to have an unexplained osmolal gap.