What is osmolar gap?
What is osmolar gap? It is the difference between measured vs calculated osmolality. This free osmolal gap calculator estimates unmeasured serum osmoles — the same clinical question answered by MDCalc-style serum osmolality / osmolarity tools. A rising gap is a classic early clue in osmolar gap toxic alcohol ingestion workups.
Measured vs calculated osmolality
Gap = measured lab osmolality − expected osmoles from Na, glucose, and urea/BUN.
Osmolar gap normal range
Use the same labs drawn close together; ethanol changes the expected gap.
Osmolar gap formula & calculated osmolality
How to calculate osmolar gap: first compute expected osmoles with the calculated osmolality formula, then subtract from the freezing-point depression lab value. This page uses the same bedside math clinicians expect from serum osmolality estimators.
Osmolar gap = Measured Osm − Calc Osm. Glucose and BUN in mg/dL; ethanol optional.
SI (mmol/L): Calc Osm = 2×Na + Glucose + BUN (± Ethanol). Unit toggles convert automatically so osmolar gap calculation with ethanol stays accurate.
Osmolality vs osmolarity
Osmolality vs osmolarity: osmolality is osmoles per kilogram of water (what the lab measures). Osmolarity is osmoles per liter (what formulas estimate). In serum they are nearly interchangeable for bedside gap math — which is why tools are often labeled serum osmolality / osmolarity.
Osmolar gap calculation example
Worked osmolar gap calculation example (Na 140, BUN 14 mg/dL, glucose 90 mg/dL, measured Osm 320, no ethanol):
Example · high gap without ethanol
- Calc Osm = 2×140 + 90/18 + 14/2.8 = 280 + 5 + 5 = 290
- Measured Osm = 320 mOsm/kg
- Gap = 320 − 290 = 30 mOsm/kg (elevated)
Takeaway: a gap of 30 without explained ethanol warrants urgent toxic-alcohol differential diagnosis.
-
1
Enter Na, BUN, glucose
Use concurrent chemistry values. Toggle mg/dL or mmol/L as needed.
-
2
Add ethanol if known
Include measured ethanol so drinking alcohol does not fake a toxic gap.
-
3
Enter measured serum osm
Compare measured vs calculated osmolality and read the gap band.
High osmolar gap causes & differential diagnosis
What does high osmolar gap mean? Unmeasured osmoles are present. Common high osmolar gap causes in the osmolar gap differential diagnosis:
Gap vs anion gap over time (toxic alcohols)
Methanol & ethylene glycol osmolar gap
Methanol osmolar gap and ethylene glycol osmolar gap are early markers of parent alcohol burden. Later, formate or glycolate drive high anion gap metabolic acidosis. Isopropyl alcohol raises the osmolar gap with ketosis but usually without severe metabolic acidosis. Always pair this calculator with the anion gap calculator for a complete toxic workup.
Clinical pearl. A normal anion gap does not rule out early toxic alcohol ingestion when the osmolar gap is large. Conversely, late presentation can show acidosis with a falling gap.
Urine osmolal gap calculator (related concept)
A urine osmolal gap calculator is a different tool: it estimates urinary ammonium (urine osm − 2×(UNa+UK) − Urea − Glucose) for metabolic acidosis / RTA questions. This page is the serum osmolar gap calculator / osmolal gap calculator for measured serum osmolality versus calculated osmolarity.
Related clinical calculators
Continue with the anion gap calculator, corrected sodium calculator, BUN creatinine ratio calculator, eGFR calculator, and FeNa calculator.
FAQ
What is osmolar gap?
The osmolar (osmolal) gap is measured serum osmolality minus calculated osmolality — an estimate of unmeasured osmoles such as alcohols.
How to calculate osmolar gap?
Compute calculated osmolarity from Na, glucose, and BUN (± ethanol), then subtract from measured serum osmolality.
What is the osmolar gap formula?
Gap = Measured Osm − Calculated Osm. US Calc Osm = 2×Na + Glucose/18 + BUN/2.8 (± EtOH/4.6).
What is the calculated osmolality formula?
US units: 2×Na + Glu/18 + BUN/2.8. SI mmol/L: 2×Na + Glu + BUN (± ethanol).
What is the osmolar gap normal range?
Typically <10 mOsm/kg. 10–15 is borderline; larger gaps suggest unmeasured osmoles.
What is osmolality vs osmolarity?
Osmolality is per kg water (measured); osmolarity is per liter (calculated). Serum values are close enough for gap bedside use.
How does osmolar gap calculation with ethanol work?
Add Ethanol(mg/dL)/4.6 to calculated osmolarity so ethanol does not inflate the unexplained gap.
What does high osmolar gap mean?
Unmeasured osmoles are present — consider ethanol (if omitted), methanol, ethylene glycol, isopropyl alcohol, mannitol, or artifact.
How do methanol and ethylene glycol affect osmolar gap?
Both raise the early osmolar gap as parent alcohols; later anion gap acidosis may rise as the osmolar gap falls.
Is a urine osmolal gap calculator the same tool?
No. Urine osmolal gap estimates urinary ammonium. This tool is serum measured vs calculated osmolality.
