Calculated Serum Osmolality CalculatorFree online fitness & health calculator — instant results, no sign-up
Estimate serum osmolality from sodium, glucose and blood urea nitrogen.
For general information only. This is not medical advice, diagnosis, dosing guidance or a treatment recommendation. Talk to a doctor or other qualified health professional about your health, pregnancy, or diet.
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About the Calculated Serum Osmolality Calculator
The Calculated Serum Osmolality Calculator estimates serum osmolality from sodium, glucose and blood urea nitrogen (BUN). It uses sodium in mmol/L and glucose and BUN in mg/dL to report an estimated concentration of dissolved particles in mOsm/kg. This tool provides an educational estimate for adults, not a direct laboratory measurement or guidance for changing fluid or salt intake.
- Sodium (mmol/L)
- The serum sodium concentration in millimoles per liter, with a default value of 140 mmol/L.
- Glucose (mg/dL)
- The blood glucose concentration in milligrams per deciliter, with a default value of 90 mg/dL.
- Blood urea nitrogen (mg/dL)
- The concentration of nitrogen contained in blood urea, reported in milligrams per deciliter, with a default value of 14 mg/dL.
Formula and method
Osm_calc = 2 × Na + Glu / 18 + BUN / 2.8
- Osm_calcCalculated serum osmolality, conventionally reported in mOsm/kg.
- NaSerum sodium concentration in mmol/L; the factor of 2 approximates the contribution of sodium and its accompanying anions.
- GluBlood glucose concentration in mg/dL; division by 18 converts it to its approximate osmolar contribution.
- BUNBlood urea nitrogen concentration in mg/dL; division by 2.8 converts it to the approximate contribution from urea.
Double the sodium value, divide glucose by 18, divide BUN by 2.8, and add the three contributions. With the default inputs, 2 × 140 + 90 / 18 + 14 / 2.8 = 280 + 5 + 5 = 290 mOsm/kg.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Sodium140 mmol/L
- Glucose90 mg/dL
- Blood urea nitrogen14 mg/dL
Results
- Calculated osmolality290 mOsm/kg
- NoteEducational estimate for adults; not a diagnosis or treatment recommendation.
How to use the Calculated Serum Osmolality Calculator
- 1Check that the laboratory results use sodium in mmol/L and glucose and BUN in mg/dL, ideally from the same blood collection.
- 2Enter the sodium concentration; the default is 140 mmol/L.
- 3Enter the glucose concentration; the default is 90 mg/dL.
- 4Enter the blood urea nitrogen concentration; the default is 14 mg/dL, and this must be BUN rather than total urea.
- 5Run the calculation and read both the calculated osmolality and the educational note.
Reading your result
The 'Calculated osmolality' row shows the estimated serum osmolality: the default inputs produce 290 mOsm/kg. A commonly used adult reference interval for measured serum osmolality is approximately 275–295 mOsm/kg, but laboratory intervals vary and a calculated result is not interchangeable with a measured result. The 'Note' row states: 'Educational estimate for adults; not a diagnosis or treatment recommendation.' A result within a typical interval does not by itself establish normal hydration or exclude illness.
Limitations
This equation approximates osmolality using the major routinely measured contributors and assumes the specified input units. It does not account for all dissolved substances, including ethanol, toxic alcohols or mannitol, and may differ from directly measured serum osmolality. It also does not measure tonicity, because urea contributes to osmolality but generally not to sustained effective osmotic pressure across cell membranes. Interpretation requires clinical context, especially with substantial electrolyte or glucose abnormalities or kidney disease. Do not use this adult educational estimate alone to diagnose dehydration, assess poisoning, or choose fluid, electrolyte or medication treatment.
Common questions
Why do the default values produce 290 mOsm/kg?+
Sodium contributes 2 × 140 = 280, glucose contributes 90 / 18 = 5, and BUN contributes 14 / 2.8 = 5. Adding these contributions gives 290 mOsm/kg.
Can I enter glucose in mmol/L or a total urea result?+
Not directly. This calculator requires glucose in mg/dL and BUN in mg/dL. Glucose in mmol/L can be converted to mg/dL by multiplying by approximately 18; total urea is not the same quantity as BUN, so confirm the analyte and convert it appropriately before entering it.
Does this calculator provide an osmolal gap?+
No. An osmolal gap requires a separately measured serum osmolality and is calculated as measured osmolality minus calculated osmolality. This tool supplies only the calculated component; gap interpretation depends on the equation, laboratory method and clinical context.
Can the result tell me how much water or salt to consume?+
No. Calculated serum osmolality alone cannot determine fluid or salt needs, and a typical result does not rule out dehydration or other problems. Intake or treatment decisions require additional information such as symptoms, fluid losses, kidney function and other laboratory results.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.