Calculated plasma osmolality estimates the concentration of dissolved particles in blood from three routine labs — sodium, glucose, and BUN — without waiting for a send-out osmometer result. This calculator applies the standard emergency-medicine formula and classifies the result against the normal 275-295 mOsm/kg range.

How it works

Calculated plasma osmolality uses Osm = 2 x Sodium + Glucose/18 + BUN/2.8. Sodium is doubled because it is accompanied by an anion (mainly chloride or bicarbonate) that contributes roughly an equal number of osmotically active particles. Glucose and BUN are divided by their respective molecular-weight-derived factors (18 and 2.8) to convert from mg/dL to mmol/L, putting all three terms on the same molar basis before summing.

The result approximates what a lab osmometer would measure directly by freezing-point depression. In a healthy person with normal renal function, calculated and measured osmolality typically agree within about 10 mOsm/kg.

Inputs and what they mean

Sodium (mEq/L) is the dominant driver of plasma osmolality under normal conditions, since it and its accompanying anions make up the majority of plasma solute.

Glucose (mg/dL) becomes an important contributor when markedly elevated, as in diabetic ketoacidosis or hyperglycemic hyperosmolar state, where it also acts as an effective osmole that draws water out of cells.

BUN (mg/dL) rises with renal impairment or dehydration. Because urea crosses cell membranes freely, it raises calculated osmolality without producing the same degree of true tonicity or fluid shift as sodium or glucose.

Limits and edge cases

This calculator computes calculated osmolality, not measured osmolality — the two can diverge when unmeasured osmotically active substances are present. An osmolal gap (measured minus calculated) greater than roughly 10 mOsm/kg is a key clue for toxic alcohol ingestion (methanol, ethylene glycol, isopropyl alcohol) and should prompt a dedicated osmolal gap calculation and clinical correlation.

Some published variants add an ethanol/3.7 term to account for reported alcohol use; this calculator does not include it, so a patient with a high blood alcohol level will show an artificially elevated osmolal gap if compared against a measured value without that adjustment. Because BUN is an ineffective osmole, calculated osmolality can overstate true tonicity in patients with significant azotemia — interpret alongside the full clinical picture, not as a standalone diagnostic test.