Serum Osmolality Calculator
Required values
mmol/L (mEq/L)
mg/dL
mg/dL
Optional — for Osmol Gap
mOsm/kg — from lab (osmometer)
mg/dL — corrects for alcohol contribution
ℹ️ Enter measured osmolality from lab to calculate osmol gap. Osmol gap >10 mOsm/kg suggests unmeasured osmoles (toxic alcohols, mannitol, propylene glycol).
Calculated Osm
—
mOsm/kg
Osmol Gap
N/A
mOsm/kg
Normal Range
275–295
mOsm/kg
Osmolality Formula & Osmol Gap
Calculated Osmolality = 2×Na + (BUN÷2.8) + (Glucose÷18) + (Ethanol÷4.6)
Normal range: 275–295 mOsm/kg
Osmol Gap = Measured Osmolality − Calculated Osmolality
Normal osmol gap: <10 mOsm/kg (some labs <15)
Normal range: 275–295 mOsm/kg
Osmol Gap = Measured Osmolality − Calculated Osmolality
Normal osmol gap: <10 mOsm/kg (some labs <15)
Causes of Elevated Osmol Gap (>10 mOsm/kg)
- Toxic alcohols: Methanol, ethylene glycol, isopropanol, propylene glycol — most important cause to exclude
- Ketoacidosis: DKA, alcoholic ketoacidosis — acetone is osmotically active
- Renal failure: Accumulation of unmeasured solutes
- Mannitol infusion: Common in neurosurgery/neurology
- Severe hyperlipidaemia or hyperproteinaemia: Pseudohyponatraemia
- Glycine, sorbitol: Irrigation fluid absorption (TURP syndrome)
High Osmol Gap + High Anion Gap Metabolic Acidosis
The combination of elevated osmol gap AND elevated anion gap metabolic acidosis should immediately raise suspicion for methanol or ethylene glycol poisoning. Both are initially osmotically active (elevating osmol gap) but are subsequently metabolised to toxic acids (formate from methanol → anion gap acidosis; oxalate from ethylene glycol → anion gap acidosis). The osmol gap may normalise as the parent alcohol is metabolised — leaving only the anion gap acidosis. Do not be falsely reassured by a normal osmol gap late in the course.
Causes of Hyperosmolality (>295 mOsm/kg)
- Hypernatraemia (most common) — water deficit, diabetes insipidus
- Hyperglycaemia — hyperosmolar hyperglycaemic state (HHS)
- Uraemia — elevated BUN in acute/chronic kidney failure
- Toxic alcohol ingestion
- Mannitol infusion
Related Calculators
⚠ Medical Disclaimer: Suspected toxic alcohol poisoning is a medical emergency. Do not wait for confirmation before initiating treatment. Contact Poison Control, give fomepizole or ethanol antidote empirically, and arrange haemodialysis. Osmol gap has significant limitations — a normal gap does not exclude toxic ingestion at late presentation.