For healthcare professionals
Serum Osmolality and Osmolal Gap Calculator
Calculated serum osmolality estimates the concentration of dissolved particles in blood from sodium, glucose and blood urea nitrogen. When a measured osmolality is also available, the difference is the osmolal gap, which points to unmeasured osmoles such as toxic alcohols. This tool is written for clinicians and is not a diagnosis.
Laboratory values
Use values from the same blood sample. The measured osmolality is optional. Nothing you enter leaves this page.
What you will see
The calculated osmolality with the working and, if you enter a measured value, the osmolal gap against the 10 mOsm/kg cut point.
Educational only, not medical advice. Not a diagnosis. Decisions belong to the treating team.
Source
The formula and the 10 mOsm/kg cut point as printed in two open-access (CC BY) case reports with literature reviews: Gabbay JM and colleagues, Clinical Case Reports 2023, and Ahmed M and colleagues, Cureus 2020. The tests check the formula, that a gap of exactly 10 is not called elevated, and that a value just above or below 10 is never shown as 10.0.
Validated in
A formula, not a validated score. The cut point is used in adult and child case reports (Ahmed 2020: an adult with ethylene glycol poisoning; Gabbay 2023: a child with leukemia and pseudohyponatremia).
Limits
A normal osmolal gap does not rule out a toxic alcohol: ethylene glycol poisoning has been reported with a gap of 12 mOsm/kg, close to normal (Ahmed 2020). A raised gap is not specific: ketoacidosis, lactic acidosis and chronic kidney disease can also raise it (Ahmed 2020; Elshafei 2020), and so can very high lipids (Gabbay 2023). This formula leaves out potassium and ethanol, and uses US units (mg/dL).
The formulas
- Calculated osmolality (mOsm/kg) = 2 x sodium (mmol/L) + glucose (mg/dL) / 18 + BUN (mg/dL) / 2.8 (Gabbay 2023) (Ahmed 2020).
- Osmolal gap = measured osmolality - calculated osmolality (Gabbay 2023).
An osmolal gap of more than 10 mOsm/kg is called elevated (Ahmed 2020) (Gabbay 2023).
Worked example: sodium 140 mmol/L, glucose 90 mg/dL and BUN 14 mg/dL give 2 x 140 + 90/18 + 14/2.8 = 280 + 5 + 5 = 290 mOsm/kg. With a measured osmolality of 310 mOsm/kg, the gap is 20 mOsm/kg, elevated.
What the case reports show
Ethylene glycol poisoning usually presents with a high anion gap metabolic acidosis and a raised osmolal gap, but the gap can be close to normal, which can delay the diagnosis; in one reported case it was 12 mOsm/kg (Ahmed 2020). In a child with leukemia, a gap above 10 mOsm/kg pointed to excess lipids 24 hours before the sample was reported as lipemic (Gabbay 2023). Very high lactic acid, as in metformin-associated lactic acidosis, can also cause an osmolal gap (Elshafei 2020).
Related tools
For a metabolic acidosis, the anion gap calculator gives the anion gap and the albumin-corrected gap, and the urine osmolal gap calculator estimates urine ammonium in a normal anion gap acidosis. For a high sodium, see the free water deficit calculator. To correct sodium for high glucose, see the corrected sodium calculator. For acute kidney injury, see the FENa calculator. Other bedside scores are listed under For healthcare professionals.
Sources
- Gabbay JM, Place AE, Ilowite M, Zhu J. Use of the osmolal gap in diagnosing mixed physiology hyponatremia in a child with B-cell acute lymphoblastic leukemia. Clin Case Rep. 2023;11(6):e7428. doi:10.1002/ccr3.7428. PMID 37255617. Open access (CC BY). Text (calculated osmolality 2[Na] + BUN/2.8 + glucose/18; gap = measured - calculated; elevated above 10 mOsm/kg; lipids). pmc.ncbi.nlm.nih.gov/articles/PMC10225615/
- Ahmed M, Janikowski C, Huda S, Ahmad A, Morrow L. Ethylene glycol poisoning with a near-normal osmolal gap: a diagnostic challenge. Cureus. 2020;12(12):e11937. doi:10.7759/cureus.11937. PMID 33425517. Open access (CC BY). Case (gap 12 mOsm/kg; normal under 10) and Discussion (formula; elevated if more than 10; ketoacidosis, lactic acidosis and chronic kidney disease as other causes). pmc.ncbi.nlm.nih.gov/articles/PMC7785502/
- Elshafei MN, Alamin M, Mohamed MFH, et al. Osmolar-gap in the setting of metformin-associated lactic acidosis: case report and a literature review. Medicine (Baltimore). 2020;99(41):e22492. doi:10.1097/MD.0000000000022492. PMID 33031284. Open access (CC BY). Text (very high lactic acid can lead to an osmolar gap). pmc.ncbi.nlm.nih.gov/articles/PMC7544303/
Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.