Serum Osmolality Calculator
Calculated serum osmolality and osmolal gap.
Enter your values to see the result.
Calculated osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 (+ ethanol ÷ 3.7) in conventional units, or 2 × Na + glucose + urea (+ ethanol) in mmol/L. An osmolal gap above about 10 suggests unmeasured osmoles such as toxic alcohols. For education and clinical decision support only. Results do not replace professional judgment, local protocols or the product information, so always verify doses and values before acting on them.
Continue your work
- Anion Gap Calculator — Check the anion gap as well.
- Corrected Sodium Calculator — Correct sodium for high glucose.
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Clear instructions
Find steps, examples and limitations below.
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How to use the Serum Osmolality Calculator
Calculated serum osmolality and osmolal gap.
- 1 Choose conventional or SI units.
- 2 Enter sodium, glucose and urea (BUN), plus ethanol if present.
- 3 Add the measured osmolality to get the osmolal gap.
Example and practical tips
Na 140, glucose 100 mg/dL and BUN 14 mg/dL give a calculated osmolality of 290.6 mOsm/kg.
Frequently asked questions
What does a raised osmolal gap mean?
A gap above about 10 mOsm/kg suggests unmeasured osmoles such as methanol, ethylene glycol, isopropanol or mannitol.
Why include ethanol?
Ethanol is a common cause of a raised gap. Including it shows whether another toxin could explain what is left.
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