Serum osmolality calculator
Enter sodium, glucose and BUN, get the calculated osmolality.
How is serum osmolality calculated?
Calculated osmolality = 2 × sodium + glucose ÷ 18 + BUN ÷ 2.8, with sodium in mEq/L and glucose and blood urea nitrogen in mg/dL (Merck Manual).
Example
Sodium 140, glucose 90 and BUN 14: 280 + 5 + 5 = 290 mOsm/kg.
What is a normal value?
MedlinePlus gives a normal measured serum osmolality of 275 to 295 mOsm/kg; ranges vary a little between labs.
What is the osmolal gap?
Measured osmolality minus the calculated value. The Merck Manual says a gap of more than about 10 suggests another osmotically active substance, such as alcohol, in the blood. With a measured 305 in the example, the gap is 15.
Can I use SI units?
Choose mmol/L for glucose and urea: the formula becomes 2 × Na + glucose + urea, all in mmol/L.
Why do the terms look like that?
Each term counts particles per liter of serum. Sodium is doubled to account for the anions that travel with it. Glucose in mg/dL is divided by 18 because 1 mmol/L of glucose is about 18 mg/dL, and blood urea nitrogen is divided by 2.8 because 1 mmol/L of urea carries about 2.8 mg/dL of nitrogen. The three terms are therefore the SI form 2 × Na + glucose + urea, written for the units labs report in the United States. Other substances in blood, such as ethanol, are not in the formula, which is what makes the osmolal gap informative.
Worked example: low sodium, high glucose
Take a sodium of 130 mEq/L, glucose 600 mg/dL and BUN 20 mg/dL. Sodium contributes 2 × 130 = 260. Glucose contributes 600 ÷ 18 = 33.3. BUN contributes 20 ÷ 2.8 = 7.1. The sum is 300.5 mOsm/kg, slightly above the 275–295 range, even though the sodium is low. Compare the first example, with normal glucose and sodium: 290 mOsm/kg. A low sodium does not by itself mean low osmolality: glucose adds particles, and the corrected sodium calculator shows how the high glucose also pulls water into the blood and lowers the sodium reading.
How do labs differ in what is normal?
MedlinePlus quotes 275 to 295 mOsm/kg for measured serum osmolality. Other laboratories print slightly different limits: for instance, a 2026 case report in EJIFCC quoted a reference interval of 275 to 300 mOsm/kg for its own lab. Use the reference range printed on your own report, and note that calculated and measured values come from different methods, so they are expected to differ a little.
Common mistakes with serum osmolality
Mixing units: this formula expects glucose and BUN in mg/dL. Typing glucose of 5.5 mmol/L and urea of 5 mmol/L into the mg/dL fields would return 282 mOsm/kg instead of 291. Entering urea instead of BUN: they are different measures, and the divisor of 2.8 is for BUN. Calling a high calculated value a diagnosis: it only says that particles are high and does not say why. And forgetting the gap: the formula leaves out alcohols and some other substances, so comparing it with a measured osmolality can reveal them.
Sources and related calculators
Sources: Merck Manual (calculated osmolality and osmolal gap); MedlinePlus, serum osmolality test (reference range); “The Truth Lies Within the Gap,” EJIFCC (2026) case report, for an example of a laboratory reference interval. Related: corrected sodium calculator, anion gap calculator and maintenance fluid calculator.
For general information only — not medical advice. Talk with your doctor or another health professional about your own health. How we check our calculators.