Serum Osmolality Calculator
Calculate serum (plasma) osmolality from sodium, glucose and urea, in conventional or SI units — and get the osmolal gap when you enter a measured value.
Add ethanol (optional)
Ethanol in mg/dL is divided by 3.7 and added to the calculated osmolality.
For education and clinical decision support only. This calculator is intended for students and clinicians and does not replace professional medical judgement, a full clinical assessment, or your laboratory's own reference ranges and units. Always confirm values and interpretation in the clinical context.
What is serum osmolality?
Serum osmolality (also written plasma osmolality) measures the concentration of dissolved particles — mainly sodium and its accompanying anions, glucose and urea — in the blood. It is a core measure of the body's water balance, tightly controlled by antidiuretic hormone (ADH) and thirst, and it underpins the assessment of hyponatraemia, hypernatraemia, and suspected toxic alcohol poisoning.
Serum osmolality formula
The widely used calculated osmolality equation in conventional units (sodium in mEq/L, glucose and BUN in mg/dL) is:
Osmolality = 2 × Na⁺ + (glucose ÷ 18) + (BUN ÷ 2.8)
In SI units, where glucose and urea are already in mmol/L, the division factors disappear:
Osmolality = 2 × Na⁺ + glucose + urea
The factor of 2 for sodium accounts for the accompanying anions (mostly chloride and bicarbonate). Dividing glucose by 18 and BUN by 2.8 simply converts mg/dL to mmol/L. Use the unit toggle above to switch between the two conventions — a result of about 290 is expected in health either way.
Normal serum osmolality range
Normal serum osmolality is approximately 275–295 mOsm/kg. Because osmolality is so tightly regulated, small sustained changes are meaningful: a high value points towards water deficit or added osmoles, while a low value suggests water excess (as in the dilutional hyponatraemias).
The osmolal gap
The osmolal gap is the difference between the value a laboratory osmometer measures and the value this equation calculates:
Osmolal gap = measured osmolality − calculated osmolality
A normal osmolal gap is roughly less than 10 mOsm/kg. A raised gap means there are unmeasured osmotically active particles in the blood that the formula does not account for. Important causes include the toxic alcohols methanol and ethylene glycol, as well as isopropyl alcohol, mannitol, glycine, acetone and severe ketoacidosis. In the right clinical setting, a high osmolal gap is an early clue to poisoning, often before the anion gap rises.
Osmolality vs osmolarity
These terms are closely related and often used interchangeably, but they are defined differently:
- Osmolality — osmoles per kilogram of solvent (mOsm/kg). This is what an osmometer measures by freezing-point depression.
- Osmolarity — osmoles per litre of solution (mOsm/L). This is what the equation technically calculates.
In dilute biological fluids like plasma the difference is tiny, so "calculated osmolarity" and "calculated osmolality" are treated as effectively equal in day-to-day practice.
Worked example
For a patient with Na⁺ 140 mEq/L, glucose 90 mg/dL and BUN 14 mg/dL: Osmolality = 2 × 140 + 90 ÷ 18 + 14 ÷ 2.8 = 280 + 5 + 5 = 290 mOsm/kg — squarely within the normal range.
Frequently asked questions
What is the serum osmolality formula?
The most common calculated serum osmolality formula is Osmolality = 2 × Na⁺ + glucose ÷ 18 + BUN ÷ 2.8 using conventional units (Na in mEq/L, glucose and BUN in mg/dL). In SI units it becomes Osmolality = 2 × Na⁺ + glucose + urea, with all values in mmol/L.
What is the normal serum osmolality range?
Normal serum osmolality is approximately 275–295 mOsm/kg. Values are tightly regulated by antidiuretic hormone and thirst, so persistent deviations are clinically important.
What is the osmolal gap?
The osmolal gap is the difference between the measured (laboratory) osmolality and the calculated osmolality: Osmolal gap = measured − calculated. A normal gap is roughly less than 10 mOsm/kg. A raised gap suggests unmeasured osmoles such as methanol, ethylene glycol, isopropyl alcohol, mannitol or acetone.
What is the difference between osmolality and osmolarity?
Osmolality is the number of osmoles per kilogram of solvent (mOsm/kg) and is what a laboratory osmometer measures. Osmolarity is per litre of solution (mOsm/L) and is what the equation calculates. In dilute body fluids the two are numerically very close, so the terms are often used interchangeably.
Should ethanol be included in the osmolality calculation?
Ethanol is an osmotically active molecule, so when a blood alcohol level is known it can be added (ethanol in mg/dL ÷ 3.7). Including it gives a more accurate calculated osmolality and prevents a known alcohol level from falsely widening the osmolal gap.
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