Serum osmolality and osmolal gap calculator
This tool estimates the concentration of dissolved particles in blood and compares it with the measured value to flag unmeasured substances.
Loading the tool…
Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter glucose and urea units, sodium, glucose, urea or bun and measured osmolality (optional).
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Conventional units: 2 × Na + glucose ÷ 18 + BUN ÷ 2.8, with glucose and BUN in mg/dL. SI units: 2 × Na + glucose + urea, all in mmol/L. Gap = measured - calculated.
Example
Inputs: Glucose and urea units = mg/dL (glucose and BUN), Sodium = 140 mmol/L, Glucose = 90, Urea or BUN = 14. Result: Calculated osmolality: 290 mOsm/kg.
Limits and caveats
- Other versions use slightly different constants, so the number can differ from your lab by a few units.
- Interpreting the osmolal gap needs clinical context and does not confirm poisoning by itself.
- An educational estimate that does not replace assessment by a doctor or pharmacist.
Questions about this tool
What is the normal range?
About 275 to 295 mOsm/kg in most references.
What is the osmolal gap for?
A gap above about 10 can point to an unmeasured substance in the blood such as methanol or ethylene glycol.
Why divide by 18 and 2.8?
To turn mg/dL into mmol/L: glucose has a molar mass of 180 and the nitrogen in urea weighs 28.