Serum Osmolality Calculator (Calculated + Osmolal Gap)
Calculated serum osmolality estimates plasma osmolality from sodium, glucose and urea (BUN). In US units it is 2 × sodium + glucose ÷ 18 + BUN ÷ 2.8. Comparing it to a measured value gives the osmolal gap, which can flag toxic alcohols.
Quick answer
Calculated serum osmolality estimates plasma osmolality from sodium, glucose and urea. In US units it is 2 × sodium + glucose ÷ 18 + BUN ÷ 2.8; in SI units it is 2 × sodium + glucose + urea (mmol/L). A normal calculated value is about 275–295 mOsm/kg. The osmolal gap is the measured value minus this calculated value.
Serum osmolality inputs
How to use the Serum Osmolality calculator
- 1Enter sodium, glucose and urea. Enter sodium, glucose and urea/BUN. Choose US units (glucose and BUN in mg/dL) or SI units (all in mmol/L).
- 2Read the calculated osmolality. The tool applies the appropriate formula and shows the calculated osmolality with the typical 275–295 mOsm/kg reference range.
- 3Optionally add a measured value. If you enter a measured (lab) osmolality, the tool also reports the osmolal gap (measured − calculated), which can flag unmeasured osmoles such as toxic alcohols.
Medical disclaimer: This is general health information, not medical advice. It does not diagnose or treat any condition, and the results are estimates based on public reference formulas. Always consult a qualified doctor about your individual health. If you think you may have a medical emergency, contact your local emergency services immediately.
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Book a consultationFrequently asked questions
What is a normal calculated serum osmolality?⌄
A normal calculated serum osmolality is roughly 275–295 mOsm/kg, though laboratory reference ranges vary slightly.
What is the osmolal gap?⌄
The osmolal gap is the measured osmolality minus the calculated osmolality. A raised gap (commonly cited as above about 10 mOsm/kg) can suggest unmeasured osmoles such as ethanol, methanol or ethylene glycol — a clinician interprets it urgently.
Which osmolality formula does this use?⌄
In US units it uses 2 × sodium + glucose ÷ 18 + BUN ÷ 2.8 (Dorwart & Chalmers). In SI units (all mmol/L) it uses 2 × sodium + glucose + urea. Several published formulas exist and can differ by a few mOsm/kg.
Sources & validation
This calculator reproduces the published Serum Osmolality score, validated for calculated serum osmolality and osmolal gap.
- Dorwart WV, Chalmers L. Comparison of methods for calculating serum osmolality from chemical concentrations, and the prognostic value of such calculations (Clin Chem 1975;21:190-194) — verified 2026-06-18
- Smithline N, Gardner KD. Gaps — anionic and osmolal (JAMA 1976;236:1594-1597) — defines the osmolal gap (measured − calculated osmolality) — verified 2026-06-18
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