What inputs does the calculator require?

It requires sodium, glucose, and BUN or urea. Sodium is entered in mmol/L.

Which formulas are used?

Conventional units use 2 × Na + Glucose/18 + BUN/2.8. SI units use 2 × Na + Glucose + Urea.

What is the approximate normal range?

The approximate normal range is 275–295 mOsm/kg.

Can ethanol be included?

Yes. Enter ethanol in mg/dL under conventional units to add the EtOH/4.6 term.

Is this a measured serum osmolality result?

No. This tool calculates an estimate from entered solute values and is not medical advice.

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Serum Osmolality Calculator (Calculated)

Calculate serum osmolality from routinely measured solutes. Conventional formula Osm = 2 × Na + Glucose/18 + BUN/2.8 (Na in mmol/L, glucose & BUN in mg/dL), or the SI-unit form Osm = 2 × Na + Glucose + Urea (all mmol/L). An optional ethanol term (EtOH/4.6) is available when a toxic-alcohol ingestion is being considered. Normal range ≈ 275–295 mOsm/kg. Values < 275 suggest hypo-osmolality (SIADH, primary polydipsia, adrenal insufficiency, hypothyroidism); values > 295 suggest hyperosmolality (hyperglycemia/HHS, hypernatremia, uremia, mannitol, toxic alcohols — the latter also raise the osmolar gap, so measure osmolality and compute the gap). Derived from Dorwart 1975, Rasouli 2014, and MDCalc. Not medical advice.

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Normal: Na=140, Glu=110 mg/dL, BUN=14 mg/dL → Osm ≈ 291.1

Osm = 2×140 + 110/18 + 14/2.8 = 280 + 6.11 + 5 = 291.1 mOsm/kg (normal, 275–295).

{"result":"Osm_calc = 291.1 mOsm/kg (Normal osmolality). 2×Na + Glu/18 + BUN/2.8.","metadata":{"input":{"unitSystem":"conventional","sodium":140,"glucose":110,"bunOrUrea":14},"result":{"osm":291.1,"unit":"mOsm/kg","category":"Normal osmolality","method":"2×Na + Glu/18 + BUN/2.8","model":"Dorwart 1975; Rasouli 2014; MDCalc. Not medical advice."}}}

SI units: Na=140, Glucose=6.1 mmol/L, Urea=5.0 mmol/L → Osm ≈ 291.1

SI formula Osm = 2×Na + Glucose + Urea = 2×140 + 6.1 + 5.0 = 291.1 mOsm/kg — matches the conventional-unit example, validating the two formulae against each other.

{"result":"Osm_calc = 291.1 mOsm/kg (Normal osmolality). 2×Na + Glucose + Urea (SI).","metadata":{"input":{"unitSystem":"si","sodium":140,"glucose":6.1,"bunOrUrea":5},"result":{"osm":291.1,"unit":"mOsm/kg","category":"Normal osmolality","method":"2×Na + Glucose + Urea (SI)","model":"Rasouli 2014; MDCalc. Not medical advice."}}}

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6 options

SettingsAdjust formats, ranges, numbers, and modes.6

Unit SystemselectRequiredSerum Sodium Na⁺ (mmol/L)numberRequiredSerum sodium (mmol/L), always in mmol/L regardless of unit system. Physiologic range ~120–160.GlucosenumberRequiredSerum glucose. mg/dL under Conventional units, mmol/L under SI units.BUN / UreanumberRequiredBUN in mg/dL under Conventional units, or serum urea in mmol/L under SI units.Ethanol (mg/dL, optional)numberOptionalSerum ethanol in mg/dL. Optional; adds EtOH/4.6 to the calculated osmolality (conventional units).Decimal PlacesnumberOptional

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Serum Osmolality Calculator (Calculated)

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1. Normal calculated osmolality with conventional units

2. Cross-checking the result with SI units

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3. Adding ethanol in a toxicology assessment