Base calculated osmolalityExcludes ethanol
— mOsm/kgGap: —
Toxicology · Laboratory interpretation
Serum Osmolality & Gap
Calculate baseline osmolality, optionally compare a measured value, and show both traditional and empiric ethanol coefficients.
Evidence recordSources and method currentVersion 0.1 · Updated July 17, 2026
Step 1
Enter same-draw labs
Private by design. Values are calculated on this device and are not saved or sent to analytics.
Step 2
Compare calculations
With ethanol ÷ 4.6Traditional coefficient
— mOsm/kgGap: —
With ethanol ÷ 3.7Empiric coefficient
— mOsm/kgGap: —
Not a toxic-alcohol rule-out test: timing, coingestants, renal function, assay method, and metabolism can produce a misleading gap. If exposure is suspected, seek toxicology or poison-center guidance urgently.
Evidence packet
What this tool is based on
This versioned record keeps the citations, equation or decision rule, units, test coverage, intended population, and exclusions attached to the tool.
How evidence review works →Ethanol coefficient · Primary publicationPurssell et al. Derivation and validation of a formula to calculate ethanol contribution to the osmolal gap.Ann Emerg Med. 2001;38(6):653–659. PMID: 11719745 ↗External validation · Validation studyGarrard et al. Validation of ethanol contribution to the osmolal gap.Clin Toxicol. 2012;50(7):562–566. PMID: 22765358 ↗
Intended population
Patients with same-draw sodium, glucose, and BUN when calculated osmolality or an osmolal-gap estimate supports assessment.
Equation record
2Na + glucose/18 + BUN/2.8, optional measured gap, and optional ethanol contributions using divisors 4.6 and 3.7.
Unit handling
Sodium uses mEq/L; glucose, BUN, and ethanol use mg/dL; measured and calculated osmolality use mOsm/kg.
Test coverage
Base formula, measured gap, two ethanol coefficients, optional-input behavior, and invalid inputs.
Do not skip
Limitations and exclusions
- The osmolal gap is not a rule-out test for toxic alcohol exposure.
- Timing, metabolism, coingestants, renal function, and laboratory method alter interpretation.
- Use contemporaneous laboratory values and the local laboratory's measured-osmolality method.
- Suspected toxic exposure requires urgent expert or poison-center consultation; this tool does not recommend treatment.
One clearly labeled, contextually relevant sponsor may support this reference area. Sponsorship will never interrupt inputs, obscure results, influence formulas, or imply clinical endorsement.
About this tool
Serum Osmolality and Osmolar Gap Calculator
Use this serum osmolality calculator to estimate calculated osmolality and, when a measured value is entered, the osmolar gap. Optional ethanol handling is shown with two methods.
The tool labels each included term and does not infer a toxic exposure from the gap. Formula choice, timing, units, and unmeasured osmoles all affect interpretation.