Serum Osmolality Calculator
Calculated serum osmolality from sodium, glucose and urea
Equation & limitations
- Equation
- Osmolality = 2 × Na + glucose/18 + BUN/2.8 (mg/dL units)
- Version
- Standard calculated-osmolality formula (mg/dL units)
- Source
- Defined by Pharmapedia for this tool; standard textbook formula.
- Appropriate in
- Adults where measured osmolality is not available.
- Limitations
- Only sodium, glucose and urea are included. Ethanol, methanol and other solutes are not captured.
- The osmolar GAP needs a measured osmolality and is not computed by this tool.
- If glucose/BUN are entered in mmol/L the result is wrong — this formula is mg/dL-specific.
For educational use. Results support, and do not replace, clinical judgement — check them against the patient and local guidelines.