Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
Osmolar gap
29.1
Calculated osmolality
290.9
Interpretation
Elevated — consider toxic alcohol or other unmeasured osmoles
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How to use this
- 1Enter measured serum osmolality (mOsm/kg).
- 2Enter sodium (mEq/L).
- 3Enter glucose (mg/dL).
- 4Enter bun (mg/dL).
- 5Enter ethanol level (if known, else 0) (mg/dL).
- 6Read your osmolar gap on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
The osmolar gap is the difference between measured serum osmolality (from the lab's freezing-point depression assay) and calculated osmolality (estimated from sodium, glucose, and BUN). A gap greater than roughly 10 mOsm/kg suggests the presence of unmeasured osmotically active substances, classically toxic alcohols like methanol, ethylene glycol, isopropyl alcohol, and ethanol itself, but also mannitol, severe hyperlipidemia, or paraproteinemia. It is used as a rapid screening tool in suspected toxic alcohol ingestion when specific levels are not immediately available, since a normal osmolar gap makes significant toxic alcohol ingestion less likely (though it does not fully exclude it, especially later in the course after the alcohol has been metabolized into its more toxic, less osmotically active acid metabolites). A falsely low or normal gap late in ingestion is a well-recognized pitfall — the anion gap metabolic acidosis from the toxic metabolites may already be present even as the osmolar gap normalizes. This tool is for education and workflow support only — not medical advice. Always verify results with a clinician and use clinical judgment alongside your institution's protocols.
Worked example
Using the values the calculator loads with:
Inputs
- Measured serum osmolality: 320 mOsm/kg
- Sodium: 140 mEq/L
- Glucose: 100 mg/dL
- BUN: 15 mg/dL
- Ethanol level (if known, else 0): 0 mg/dL
Results
- Osmolar gap: 29.1
- Calculated osmolality: 290.9
- Interpretation: Elevated — consider toxic alcohol or other unmeasured osmoles
What each field means
Inputs
- Measured serum osmolality (mOsm/kg)
- The measured serum osmolality used in the calculation, measured in mOsm/kg. Starts at 320 mOsm/kg so you have a working example on load. Accepted range: 250–450 mOsm/kg.
- Sodium (mEq/L)
- The sodium used in the calculation, measured in mEq/L. Starts at 140 mEq/L so you have a working example on load. Accepted range: 110–170 mEq/L.
- Glucose (mg/dL)
- The glucose used in the calculation, measured in mg/dL. Starts at 100 mg/dL so you have a working example on load. Accepted range: 40–800 mg/dL.
- BUN (mg/dL)
- The bun used in the calculation, measured in mg/dL. Starts at 15 mg/dL so you have a working example on load. Accepted range: 2–150 mg/dL.
- Ethanol level (if known, else 0) (mg/dL)
- The ethanol level (if known, else 0) used in the calculation, measured in mg/dL. Starts at 0 mg/dL so you have a working example on load. Accepted range: 0–500 mg/dL.
Results
- Osmolar gap
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Calculated osmolality
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Interpretation
- Returned as a plain value. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
What counts as an elevated osmolar gap?
Above roughly 10 mOsm/kg is generally considered elevated, though 'normal' has real variability between labs and individuals — some healthy people run gaps up to 10-15 at baseline. Trend and clinical context matter more than a single cutoff, especially with suspected toxic ingestion.
Can a normal osmolar gap rule out toxic alcohol poisoning?
No, not reliably, especially late in the course. As methanol or ethylene glycol is metabolized into its acidic byproducts, the osmotically active parent alcohol decreases while the anion gap acidosis from the toxic metabolites increases — so a normal osmolar gap late in ingestion can coexist with severe, ongoing toxicity.
Why include ethanol in the calculated osmolality?
Ethanol is itself an osmotically active substance, and a patient who has been drinking will have an elevated measured osmolality from ethanol alone. Including a measured ethanol level in the calculation (divided by 4.6 to convert mg/dL to mOsm/kg contribution) prevents ethanol from falsely inflating the gap and pointing you toward a different toxic alcohol.
What else besides toxic alcohols raises the osmolar gap?
Severe hypertriglyceridemia, paraproteinemia (as in multiple myeloma), mannitol administration, and severe renal failure with accumulated unmeasured solutes can all raise the osmolar gap. Clinical context — ingestion history, lipid panel, and renal function — helps narrow the differential.
Accuracy and limitations
- Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
- Results do not account for medical conditions, medication, or pregnancy.
- Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.
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Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Osmolar Gap Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/osmolar-gap
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/osmolar-gap" target="_blank" rel="noopener">Osmolar Gap Calculator — RevenueLab</a> (2026).</p>
Source: [Osmolar Gap Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/osmolar-gap) (2026).