Serum Osmolality Calculator

This serum osmolality calculator estimates the concentration of osmotically active particles from sodium, glucose, blood urea nitrogen and optional ethanol. When a measured serum osmolality is entered, it also calculates the osmolal gap: measured minus calculated.

Calculated osmolality and the osmolal gap are clinical screening tools, not diagnoses. Different equations and laboratory methods produce different values, and a normal gap does not reliably exclude every toxic exposure. Suspected poisoning, altered consciousness, severe illness or another emergency needs immediate professional care or poison-centre guidance.

1. Enter sodium in mEq/L.
2. Enter BUN and glucose in mg/dL or mmol/L, selecting the actual units.
3. Enter ethanol if known, using mg/dL or mmol/L; leave the numeric field blank when unavailable.
4. Optionally enter measured osmolality in mOsm/kg from the same clinical period.
5. Calculate and review the estimate. The gap appears only when a measured value was provided.

The page converts SI inputs before applying the conventional equation. All concentrations must be nonnegative.

The estimate approximates measured serum osmolality from commonly measured solutes. The gap represents contributions not captured by the chosen equation plus ordinary analytical and biological variation. Reference limits depend on the formula, measurement method, ethanol treatment and laboratory.

An increased gap can occur with additional osmoles, including some toxic alcohols, but sensitivity changes with timing and metabolism. Other clinical findings, acid–base status and direct testing matter. A calculated number should never delay toxicology consultation or emergency treatment.

Use laboratory values obtained close together. The form’s ethanol divisor follows a published empiric relationship; other equations use other coefficients, so calculators may disagree. Document the exact equation whenever the gap is communicated.

Sodium contributes twice in this conventional estimate because its accompanying anions also contribute to particle concentration. Urea and glucose contributions depend on their molecular units, which is why the divisors are specific and cannot be exchanged. The estimate also omits many smaller solutes. A negative gap can occur and does not automatically mean the arithmetic is wrong; method bias and ordinary variation require proper laboratory context.