Serum Osmolal Gap CalculatorFree online fitness & health calculator — instant results, no sign-up
Compare measured serum osmolality with a sodium-glucose-BUN estimate.
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 Serum Osmolal Gap Calculator
The Serum Osmolal Gap Calculator compares measured serum osmolality with an estimate calculated from sodium, glucose, and blood urea nitrogen (BUN). It reports the difference in mOsm/kg and does not include ethanol in the estimate. This tool provides educational context for adult laboratory results, not a diagnosis or treatment recommendation.
- Measured osmolality (mOsm/kg)
- The laboratory-measured concentration of dissolved particles in serum, expressed in mOsm/kg, defaults to 295.
- Sodium (mmol/L)
- The serum sodium concentration, expressed in mmol/L, defaults to 140.
- Glucose (mg/dL)
- The blood glucose concentration, expressed in mg/dL, defaults to 90.
- Blood urea nitrogen (mg/dL)
- The blood urea nitrogen concentration, expressed as nitrogen in mg/dL rather than total urea, defaults to 14.
Formula and method
O_calc = 2 × Na + Glu / 18 + BUN / 2.8; Gap = O_meas − O_calc. With the default inputs: O_calc = 2 × 140 + 90 / 18 + 14 / 2.8 = 290 mOsm/kg; Gap = 295 − 290 = 5 mOsm/kg.
- O_calcEstimated serum osmolality, conventionally reported in mOsm/kg for this clinical comparison.
- NaSerum sodium concentration in mmol/L.
- GluBlood glucose concentration in mg/dL.
- BUNBlood urea nitrogen concentration in mg/dL.
- O_measMeasured serum osmolality in mOsm/kg.
- GapMeasured osmolality minus estimated osmolality, in mOsm/kg.
The calculator doubles sodium to approximate the contribution of sodium and its accompanying anions, then adds glucose divided by 18 and BUN divided by 2.8. These divisors convert the specified mass concentrations into approximate osmotic contributions. It subtracts the resulting estimate from measured osmolality; no ethanol correction is applied.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Measured osmolality295 mOsm/kg
- Sodium140 mmol/L
- Glucose90 mg/dL
- Blood urea nitrogen14 mg/dL
Results
- Osmolal gap5 mOsm/kg
- NoteThis formula excludes ethanol; interpretation depends on the laboratory method.
- NoteEducational estimate for adults; not a diagnosis or treatment recommendation.
How to use the Serum Osmolal Gap Calculator
- 1Obtain measured serum osmolality, sodium, glucose, and BUN from the same blood draw when possible.
- 2Enter measured osmolality in mOsm/kg and sodium in mmol/L.
- 3Enter glucose and BUN in mg/dL, checking that the laboratory reports BUN rather than total urea.
- 4Calculate the gap; the default inputs of 295, 140, 90, and 14 produce an estimated osmolality of 290 mOsm/kg and an osmolal gap of 5 mOsm/kg.
- 5Read both notes and compare the result with the laboratory's reference interval and clinical context.
Reading your result
The 'Osmolal gap' row reports measured minus estimated osmolality. The default result, 5 mOsm/kg, means the measured value is 5 mOsm/kg higher than the estimate; a negative result means it is lower. The first 'Note' row states: 'This formula excludes ethanol; interpretation depends on the laboratory method.' Ethanol can contribute to the measured value without being counted in this estimate. The second 'Note' row states: 'Educational estimate for adults; not a diagnosis or treatment recommendation.' A gap alone cannot establish or exclude a medical condition.
Limitations
This is an empirical adult estimate that uses concentration-based inputs to approximate osmolality, which is measured per kilogram of solvent. Results depend on the formula, laboratory methods, analytical variation, and sample timing, so there is no single cutoff appropriate for every laboratory. Ethanol and other unmeasured solutes are not included. Incorrect units or substituting total urea for BUN will distort the result. An elevated gap is nonspecific, and a normal or small gap does not rule out toxic alcohol exposure, particularly as substances are metabolized. Suspected poisoning requires urgent medical assessment rather than reliance on this calculator.
Common questions
Why do the default inputs produce an osmolal gap of 5 mOsm/kg?+
Sodium contributes 2 × 140 = 280, glucose contributes 90 / 18 = 5, and BUN contributes 14 / 2.8 = 5. The estimate is therefore 290 mOsm/kg, and subtracting it from the measured 295 mOsm/kg gives 5 mOsm/kg.
Does this calculator correct the gap for ethanol?+
No. Ethanol is not an input or a term in this formula. If ethanol is present, it can raise measured osmolality and contribute to the reported gap; clinical interpretation may require an ethanol-inclusive calculation.
Can I enter glucose in mmol/L or total urea instead of BUN?+
Not directly. This formula requires glucose in mg/dL and BUN in mg/dL. Convert glucose to the required units and confirm whether the laboratory reports urea or urea nitrogen before entering a value.
Does a gap of 5 mOsm/kg rule out toxic alcohol poisoning?+
No. Although this value falls within many laboratories' reference intervals, interpretation must use the local interval and clinical findings. Toxic alcohol exposure can occur without an elevated gap, so suspected exposure needs urgent medical assessment.
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.