Renal & electrolytes
Serum osmolality calculator
Two times sodium plus glucose plus urea — and the gap against the lab's measured value that flags a toxic alcohol early.
How to read the result
Calculated osmolality (mOsm/kg) falls into these published bands.
A drowsy patient with sodium 140, glucose 5 and urea 5 mmol/L; the laboratory reports a measured osmolality of 320 mOsm/kg.
Calculated osmolality 290 mOsm/kg; osmolar gap 30 — raised
A gap this far above 10 mOsm/kg means unmeasured osmoles, with toxic alcohols first on the list — and in suspected poisoning the gap is treated as significant early, because it falls as the parent alcohol is metabolised.
Who this is for
For emergency and ICU clinicians facing suspected toxic-alcohol ingestion, an unexplained acidosis, or a coma work-up. Methanol and ethylene glycol raise the measured osmolality before their acidic metabolites raise the anion gap, so an early osmolar gap above 10 mOsm/kg is a critical clue — this tool computes the calculated osmolality and the gap in one pass.
How it works
What it calculates
Calculated serum osmolality (2 × Na + glucose + urea, all in mmol/L) and — when a measured osmolality is entered — the osmolar gap between them.
When to use it
Suspected toxic-alcohol ingestion is the classic use: methanol and ethylene glycol raise the measured osmolality before their acidic metabolites raise the anion gap, so an early raised gap (> 10 mOsm/kg) is a critical clue.
Worth remembering
Ethanol itself raises the gap — factor it in where co-ingestion is possible — and a normal gap late in an ingestion does not exclude it, because the parent alcohol has already been metabolised.
Built on published evidence
- Fazekas et al., Intensive Care Med 2013 (equation review)
Codification last reviewed 2026-08-28, and checked against published reference values in automated tests on every release — see the full evidence ledger.
Questions clinicians ask
What is a significant osmolar gap?
Above 10 mOsm/kg is abnormal. A raised gap points to unmeasured osmoles: toxic alcohols (methanol, ethylene glycol), ethanol, mannitol, and glycine are the usual culprits.
Does a normal gap exclude toxic-alcohol ingestion?
No — late in an ingestion the parent alcohol has already been metabolised, so the osmolar gap falls while the anion-gap acidosis rises. A normal gap in a late presenter does not close the question.
Which formula does the calculator use?
Calculated osmolality = 2 × Na + glucose + urea, with everything in mmol/L as South African laboratories report — the widely used formula reviewed by Fazekas et al. (Intensive Care Med 2013). The usual serum range is about 275–295 mOsm/kg.
Does ethanol raise the osmolar gap?
Yes — ethanol itself contributes to the gap, so factor it in wherever co-ingestion is possible before attributing the whole gap to methanol or ethylene glycol.
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