Serum Osmolality Calculator

Calculate serum osmolality from sodium, glucose, and urea, and determine the osmolal gap when a measured osmolality is available — a key step in evaluating suspected toxic alcohol ingestion.

🧪 Lab Value📐 Osm = 2 × Na + glucose/18 + BUN/2.8🏥 Health
Sodium (mmol/L)
Glucose (mg/dL)
BUN (mg/dL)
Measured osmolality (optional)
Include ethanol term
Ethanol (mg/dL, if applicable)
Please enter valid values.

Formula & Reference

VariableSymbolFormulaUnits
Serum Osmolality Calculator—Osm = 2 × Na + glucose/18 + BUN/2.8mOsm/kg

Step-by-Step Examples

Example 1
Normal Result

Sodium 140, glucose 90 mg/dL, BUN 14 mg/dL.

  • Sodium term = 2 × 140 = 280
  • Glucose term = 90 / 18 = 5
  • Urea term = 14 / 2.8 = 5
  • Calculated = 290 mOsm/kg
✓ Calculated osmolality 290 mOsm/kg
Example 2
Hyperglycaemic State

Sodium 132, glucose 800 mg/dL, BUN 40 mg/dL.

  • Sodium term = 264
  • Glucose term = 800 / 18 = 44.4
  • Urea term = 40 / 2.8 = 14.3
  • Calculated = 322.7 mOsm/kg
✓ Calculated osmolality 322.7 mOsm/kg — markedly hyperosmolar
Example 3
Elevated Osmolal Gap

Sodium 138, glucose 100, BUN 12, measured osmolality 340.

  • Calculated = 276 + 5.6 + 4.3 = 285.9
  • Gap = 340 − 285.9 = 54.1 mOsm/kg
  • A gap this large suggests unmeasured osmoles
✓ Osmolal gap 54.1 — markedly elevated

Real-World Applications

☠️
Toxic Alcohol Evaluation
An elevated osmolal gap is a key early clue to methanol or ethylene glycol ingestion, often available before confirmatory levels return.
🏥
Hyperosmolar States
Calculated osmolality quantifies the severity of hyperosmolar hyperglycaemic state and guides fluid management.
🧪
Laboratory Correlation
Comparing calculated and measured osmolality is a standard step in evaluating unexplained metabolic acidosis.
📚
Toxicology Education
The osmolal gap paired with the anion gap forms a core diagnostic framework in medical toxicology teaching.

Common Mistakes to Avoid

⚠️
Relying on a normal osmolal gap to exclude poisoning

As toxic alcohols are metabolised, the osmolal gap falls while the anion gap rises. A late presentation can show a normal gap despite serious poisoning.

⚠️
Forgetting to account for ethanol

Ethanol is a potent osmole. If a patient has consumed alcohol and it is not included in the calculation, the apparent gap will be spuriously elevated.

⚠️
Treating a small gap as meaningful

The reference range for the osmolal gap is wide and varies between laboratories and formulas. Gaps under about 10 mOsm/kg carry little diagnostic weight.

Frequently Asked Questions

How is serum osmolality calculated? ▾
The most common formula doubles the sodium to account for accompanying anions, then adds glucose divided by 18 and BUN divided by 2.8 to convert mg/dL into mOsm/kg.
What is the osmolal gap? ▾
The difference between the laboratory-measured osmolality and the calculated value. A large gap implies osmotically active substances the formula does not account for.
What causes a high osmolal gap? ▾
Methanol, ethylene glycol, isopropanol, and ethanol are the classic causes, along with mannitol, glycerol, and certain severe metabolic states.
Why divide glucose by 18 and BUN by 2.8? ▾
Those divisors convert mg/dL into mmol/L using each substance's molecular weight, putting all terms into consistent osmolar units.
Is a normal osmolality reassuring? ▾
Not by itself. Osmolality can be normal in the presence of serious electrolyte or acid-base derangement, so it must be interpreted alongside the anion gap and the clinical picture.

Related Health Calculators

🧪 Corrected Sodium💊 Anion Gap⚡ Electrolyte Balance🧪 eGFR CKD-EPI💧 Hydration Needs