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- Serum osmolality measures the concentration of dissolved particles in the blood. Sodium, glucose and urea make up most of it.
- Calculated osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8 (US units), or 2 × Na + glucose + urea in mmol/L.
- The usual reference range is about 275 to 295 mOsm/kg. Each lab sets its own range.
- Add a measured osmolality to get the osmolal gap. This calculator is for education, not diagnosis.
How to use the serum osmolality calculator
Choose units
US units use mg/dL for glucose and BUN. SI units use mmol/L for glucose and urea.
Enter sodium
Sodium is in mmol/L, which is the same number as mEq/L.
Enter glucose and BUN or urea
Copy them from the lab report in the units you chose.
Add measured osmolality if you have it
The calculator then shows the osmolal gap: measured minus calculated.
Serum osmolality formula
Calculated serum osmolality
US units: 2 × Na (mmol/L) + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8
SI units: 2 × Na + glucose + urea (all in mmol/L)
Osmolal gap = measured osmolality − calculated osmolality
Sodium is doubled to account for the negative ions, mainly chloride and bicarbonate, that pair with it. Dividing by 18 and 2.8 converts glucose and BUN from mg/dL to mmol/L.
Normal values
Reference values
Typical adult values. Always use the range printed on the lab report.
- Serum osmolality about 275 to 295 mOsm/kg
- Osmolal gap commonly treated as raised above about 10 mOsm/kg
Small differences between calculated and measured values are expected, because the formula leaves out minor solutes.
Worked examples
Example 1: typical values
Sodium 140 mmol/L, glucose 90 mg/dL, BUN 14 mg/dL.
- Sodium part 2 × 140 = 280
- Glucose part 90 ÷ 18 = 5.0
- BUN part 14 ÷ 2.8 = 5.0
- Calculated osmolality 290.0 mOsm/kg
This is inside the usual reference range.
Example 2: high blood sugar
Sodium 130 mmol/L, glucose 600 mg/dL, BUN 40 mg/dL.
- Sodium part 2 × 130 = 260
- Glucose part 600 ÷ 18 = 33.3
- BUN part 40 ÷ 2.8 = 14.3
- Calculated osmolality 307.6 mOsm/kg
Very high glucose can raise osmolality even when sodium is low.
Example 3: osmolal gap
The values from Example 1 with a measured osmolality of 310 mOsm/kg.
- Calculated 290.0 mOsm/kg
- Measured 310 mOsm/kg
- Osmolal gap 310 − 290.0 = 20.0 mOsm/kg
A gap this size is above 10 and would be interpreted by a clinician with the rest of the results.
What the osmolal gap means
The formula only counts sodium, glucose and urea. If the measured osmolality is much higher than the calculated value, something else is adding particles to the blood. Clinicians use a raised gap as a clue to look for unmeasured substances, such as alcohols, alongside the history, symptoms and other tests. A normal gap does not rule anything out on its own.
Where this calculator helps
Students
Practice the formula and check homework answers.
Nurses and clinicians
A quick arithmetic check with every step shown.
Unit conversion
Switch between mg/dL and mmol/L without mistakes.
Reading a report
Understand how a calculated osmolality is reached.
This calculator is for education and checking arithmetic only. It does not diagnose or guide treatment. Lab results must be interpreted by a qualified clinician who knows the patient.
Serum osmolality calculator FAQ
How do you calculate serum osmolality?
Multiply sodium by 2, add glucose divided by 18 and BUN divided by 2.8, with glucose and BUN in mg/dL. In SI units, add 2 × sodium, glucose and urea, all in mmol/L.
What is a normal serum osmolality?
About 275 to 295 mOsm/kg in adults, though each lab publishes its own reference range.
What is the osmolal gap?
The measured osmolality minus the calculated osmolality. A gap above about 10 mOsm/kg is commonly treated as raised.
Why is sodium multiplied by 2?
Each sodium ion is paired with a negative ion, mostly chloride or bicarbonate, so doubling sodium accounts for both.
Is plasma osmolality the same as serum osmolality?
In practice the two are very close and the same formula is used for both.
What is the difference between osmolality and osmolarity?
Osmolality is per kilogram of water (mOsm/kg) and is what labs measure. Osmolarity is per liter of solution (mOsm/L). The calculated value is often used for either.