Serum osmolality and osmolar gap calculator
Enter sodium, glucose and BUN (or urea in SI units) to get the calculated serum osmolality. If the lab also measured osmolality, tick the box to see the osmolar gap. The result is for information and teaching, not a diagnosis.
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How to use it
- Choose US units (glucose and BUN in mg/dL) or SI units (glucose and urea in mmol/L).
- Enter sodium, glucose and BUN or urea from the same blood sample.
- If you have a measured osmolality, tick the box and enter it to see the osmolar gap.
- US units: 2 × Na + glucose ÷ 18 + BUN ÷ 2.8
- SI units: 2 × Na + glucose + urea
- Osmolar gap = measured − calculated; above 10 is read as raised
Worked example
Sodium 135 mmol/L, glucose 450 mg/dL and BUN 28 mg/dL: 2 × 135 + 450 ÷ 18 + 28 ÷ 2.8 = 270 + 25 + 10 = 305 mOsm/kg. With a measured osmolality of 320 the gap is 320 − 305 = 15, raised.
The default values, sodium 140, glucose 90 mg/dL and BUN 14 mg/dL, give 280 + 5 + 5 = 290 mOsm/kg.
Tips
- Take all values from the same sample; glucose and urea change within hours.
- Ethanol raises the measured osmolality and therefore the gap. A raised gap in someone who has been drinking may be explained by alcohol alone.
- In SI units enter urea, not creatinine; the two are easy to confuse on a kidney panel.
- For information and teaching only.
Frequently asked questions
What is the formula for calculated osmolality?
In US units: 2 × sodium + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8. In SI units: 2 × sodium + glucose (mmol/L) + urea (mmol/L). Sodium is doubled to account for the chloride and bicarbonate that go with it.
What is a normal serum osmolality?
Measured serum osmolality is usually about 275–295 mOsm/kg in adults. Labs give their own reference range on the report.
What does a raised osmolar gap mean?
The gap is measured minus calculated osmolality. Above about 10 mOsm/kg suggests that something the formula does not count is in the blood, such as alcohol, methanol, ethylene glycol, mannitol or acetone. Ethanol is the most common reason. A gap is a clue for the clinician, not a diagnosis.
Is osmolality the same as osmolarity?
Nearly. Osmolality is per kilogram of water (mOsm/kg), osmolarity per litre of solution (mOsm/L). Labs measure osmolality; the calculated value is used as an estimate of it.