Calculators · General Medicine · Formula

Calculated serum osmolality and osmolar gap

Also called osmolal gap, osmole gap, calculated osmolarity

Calculated osmolality from Na, glucose and urea; the gap against measured osmolality screens for unmeasured osmoles such as toxic alcohols.

mmol/L
mOsm/kg

Fill 3 more fields to see the result.

Formula and scoring

Calculated osmolality = 2 × Na + glucose + urea (all mmol/L) = 2 × Na + glucose (mg/dL)/18 + BUN (mg/dL)/2.8. Osmolar gap = measured − calculated.

Exam and ward pearls

  • Effective osmolality (tonicity) leaves out urea; the calculated value here includes it.
  • Lynd 2008: the osmole gap lacks specificity (raised in DKA, shock, alcoholic acidosis) and its wide normal range makes it insensitive to small toxic doses.

When not to rely on it

  • Dorwart 1975 found 1.86 × Na + glucose/18 + BUN/2.8 + 9 fitted best in 715 sera; each formula gives a different gap, so use your laboratory formula and range.

Source

  1. Umpierrez GE, Davis GM, ElSayed NA, et al. Hyperglycemic crises in adults with diabetes: a consensus report. Diabetes Care. 2024;47(8):1257-1275. Figure 2 and Table 2. doi:10.2337/dci24-0032 · PMID 39052901
  2. Dorwart WV, Chalmers L. Comparison of methods for calculating serum osmolality from chemical concentrations, and the prognostic value of such calculations. Clin Chem. 1975;21(2):190-4. PMID 1112025
  3. Lynd LD, Richardson KJ, Purssell RA, et al. An evaluation of the osmole gap as a screening test for toxic alcohol poisoning. BMC Emerg Med. 2008;8:5. doi:10.1186/1471-227X-8-5 · PMID 18442409

Formula and cut-offs checked against these sources on 8 Nov 2026. Educational tool for clinicians; it does not replace clinical judgement.

More General Medicine calculators