Calculate serum osmolality and the osmolal gap from common chemistry inputs. Use it as an interpretation aid when reviewing unmeasured osmoles alongside the broader clinical picture.
The Osmolality Gap Calculator compares measured serum osmolality with the value calculated from sodium, glucose, blood urea nitrogen, and optional ethanol. The difference between those numbers is the osmolal gap, which can help flag unmeasured osmotically active substances and frame the laboratory pattern more clearly.
A normal osmolal gap is often described as less than about 10 mOsm/kg, but mild elevations can occur for several reasons and do not point to one diagnosis on their own. The gap is most useful when it is interpreted alongside the anion gap, acid-base status, renal function, exposure history, and the timing of the presentation.
Use this page as a structured worksheet for calculated osmolality, ethanol-adjusted osmolality, and the resulting gap. It is meant to support interpretation of the reported numbers, not to replace toxicology testing, Poison Control guidance, or emergency evaluation when poisoning is a real concern.
The osmolal gap is a useful bedside cross-check when measured osmolality does not match the expected chemistry-based calculation. It can help you organize possible explanations such as ethanol, propylene glycol, mannitol, contrast exposure, or toxic alcohols, but the number only becomes meaningful when it is read in context rather than treated as a stand-alone diagnosis.
Calculated Osmolality = (2 × Na) + (Glucose ÷ 18) + (BUN ÷ 2.8) With ethanol: Calculated Osmolality = (2 × Na) + (Glucose ÷ 18) + (BUN ÷ 2.8) + (EtOH ÷ 4.6) Osmolal Gap = Measured Osmolality − Calculated Osmolality Interpretation bands used on this page: • Normal: < 10 mOsm/kg • Mild elevation: 10–20 mOsm/kg • Elevated: 20–40 mOsm/kg • Very high: > 40 mOsm/kg These bands are only an interpretation aid and do not identify a single cause by themselves.
Result: Calculated Osm = 290.6 mOsm/kg, Gap = 29.4 mOsm/kg — Elevated
Calculated = (2 × 140) + (100 ÷ 18) + (14 ÷ 2.8) = 280 + 5.56 + 5.0 = 290.56. Gap = 320 − 290.56 = 29.4. A gap in this range deserves review in context with acid-base status, the anion gap, ethanol level, exposure history, and the timing of presentation rather than being treated as a stand-alone poisoning diagnosis.
The osmolal gap is most useful when it is paired with the anion gap and acid-base picture. A higher osmolal gap with little anion-gap change can occur earlier in an exposure when the parent alcohol is still present. Later in the course, the osmolal gap can fall while the anion gap rises as acidic metabolites accumulate. This timing issue is one reason a normal gap does not always exclude prior exposure.
The osmolal gap is not specific to toxic alcohols. Ethanol, propylene glycol, mannitol, contrast media, and other unmeasured osmoles can all widen the gap, and some laboratories or formulas produce slightly different baselines. The page therefore treats the result as an interpretation aid rather than a diagnosis.
Use the output to compare measured and calculated osmolality in one place, document the calculation clearly, and see whether the laboratory pattern deserves more review. When poisoning is a real concern, the next steps depend on the full clinical scenario, local toxicology guidance, and confirmatory testing rather than the worksheet alone.
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This page calculates expected osmolality from sodium, glucose, blood urea nitrogen, and optional ethanol, then subtracts that value from the measured serum osmolality to produce the osmolal gap. The gap is displayed beside broad interpretation bands so the user can review whether the measured value meaningfully exceeds the chemistry-based expectation.
The result is an interpretation aid, not a poisoning diagnosis. Osmolal-gap performance changes with ethanol, timing of ingestion, laboratory method, ketoacidosis, renal failure, and other unmeasured osmoles, so the value has to be read with the anion gap and the clinical scenario.
The osmolal gap is the difference between the measured serum osmolality (by freezing point depression in the lab) and the calculated osmolality (using sodium, glucose, BUN, and ethanol). A gap >10 indicates unmeasured osmotically active substances in the blood.
Common causes include ethanol, toxic alcohols, propylene glycol, mannitol, radiographic contrast, and other unmeasured osmoles. A normal gap does not rule out a prior ingestion if the parent alcohol has already been metabolized, and an elevated gap does not prove one specific toxin without the rest of the workup.
In toxic alcohol ingestion, there's a characteristic pattern: early on, the osmolal gap is high (parent alcohol present) and anion gap is normal. As the alcohol is metabolized into acids (formic acid from methanol, glycolic acid from ethylene glycol), the osmolal gap falls and the anion gap rises. At any point, the sum of both gaps tends to remain elevated.
Osmolality is measured per kg of solvent (water) using freezing point depression. Osmolarity is calculated per liter of solution. In clinical practice, the difference is small (typically <1–2%) because blood is mostly water. The lab measures osmolality; we calculate an approximation often technically called osmolarity.
Yes. If the toxic alcohol has already been fully metabolized into acids, the parent compound is gone and the osmolal gap returns to normal. In this case, the anion gap will be elevated instead. This is why both gaps should be assessed together in suspected poisoning.
Toxic alcohol exposure is part of the differential when the clinical picture includes unexplained altered mental status, visual symptoms, severe metabolic acidosis, kidney injury, or an unexplained osmolal gap. The page is best used as one laboratory cross-check within that broader emergency evaluation rather than as a confirmation tool on its own.