Calculate serum osmolality from sodium, glucose and BUN, with the osmolar gap when a measured value is known.
Result
How to use this tool
Enter sodium in mmol/L.
Enter glucose and BUN in mg/dL.
Optionally enter a measured osmolality for the gap.
Read the calculated value and its reference reading.
Formula & worked example
Sodium 140, glucose 90 and BUN 14 give 280 + 5 + 5 = 290 mOsm/kg, within the usual 275-295 range.
Limitations — read this before you rely on it
A calculation, not a measurement; it assumes standard conversions and misses unmeasured osmoles unless a measured value is entered. This tool is for education only and is not medical advice. Talk to a clinician about any result that worries you, and never change treatment because of a calculator result.
Frequently asked questions
The concentration of dissolved particles in blood, normally about 275-295 mOsm/kg, controlling water balance.
To convert mg/dL to mmol/L using their molecular weights, since osmolality counts particles, not mass.
Measured minus calculated osmolality; a raised gap points to unmeasured substances like ethanol or toxic alcohols.
Usually within about 10, but they differ when unmeasured osmoles are present or labs vary.
Then use 2 x sodium + glucose + urea directly, without the 18 and 2.8 divisors.