Osmolal Gap Calculator

Calculate the osmolal gap between measured and estimated serum osmolality.

Aug 31, 2026 Fluids, Electrolytes & Acid Base #osmolality #osmolal-gap #toxins #alcohol

Calculate

All fields required unless noted

Measured serum osmolality (mOsm/kg)

Serum sodium (mEq/L)

Serum glucose (mg/dL)

BUN (mg/dL)

Calculating...

Enter your values and press Calculate to see the result.

Formula & Clinical Notes

Formula

Osmolal Gap = Measured Osm − (2 × Na + Glucose / 18 + BUN / 2.8)

Clinical Notes

The osmolal gap is the difference between measured and estimated serum osmolality. A gap above 10 mOsm/kg suggests unmeasured osmotically active substances, notably methanol, ethylene glycol, isopropanol, or propylene glycol. In high anion gap acidosis with an elevated osmolal gap, toxic alcohol ingestion should be considered.

Sources

StatPearls — Serum Osmolality
StatPearls — Osmometer