Is it osmolality or osmolarity?

Serum is usually measured as osmolality by an osmometer. People often search for osmolarity, but this page is framed around the clinical serum osmolality and osmolal gap workflow.

What is BUN in the formula?

BUN means blood urea nitrogen. Do not substitute a urea result without checking the unit conversion, because BUN and urea are not the same laboratory quantity.

Does a high osmolal gap diagnose poisoning?

No. It can support concern for unmeasured osmoles such as alcohols, but diagnosis depends on symptoms, acid-base status, toxicology, kidney function and clinician evaluation.

Serum Osmolality Calculator

Estimate calculated serum osmolality, effective tonicity and optional osmolal gap from sodium, glucose, BUN, ethanol and measured osmolality.

Sodium, mEq/L

Glucose

BUN / urea contribution

Enter BUN when using mg/dL. Enter urea contribution when using mmol/L; do not mix BUN and urea units.

Ethanol (optional)

Measured osmolality (optional)

Enter an osmometer result to calculate the osmolal gap. A normal gap does not rule out toxic alcohol exposure.

Calculated serum osmolality and osmolal gap

English medical search intent usually expects calculated serum osmolality, an optional measured value and the osmolal gap. The calculator keeps BUN separate from urea and treats ethanol as an optional contributor, because unit confusion can change the result.

Osm_{c a l c} = 2 N a + \frac{g l u cos e _{m g / d L}}{18} + \frac{B U N _{m g / d L}}{2.8} + \frac{e t han o l _{m g / d L}}{4.6}

Common US-unit calculated serum osmolality model.

Osmolal gap = Osm_{m e a s u r e d} - Osm_{c a l c}

Requires a measured serum osmolality from an osmometer.

Not a toxic alcohol rule-out

A normal or modest osmolal gap does not rule out toxic alcohol exposure, especially later after metabolism has begun. Altered mental status, seizure, severe acidosis or suspected toxic alcohol ingestion needs urgent medical evaluation.

Effective tonicity

effective tonicity = 2 N a + \frac{g l u cos e _{m g / d L}}{18}

Urea and ethanol are excluded from this simplified effective-tonicity estimate.

Output	How to read it
Calculated osmolality	Model-based estimate from sodium, glucose, BUN and optional ethanol.
Measured osmolality	Laboratory osmometer value, if available.
Osmolal gap	Difference between measured and calculated values; suggests unmeasured osmoles but is not diagnostic alone.
Effective tonicity	Simplified estimate of osmoles that drive water shifts across cell membranes.

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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