About serum osmolality
Serum osmolality measures the total concentration of dissolved particles (solutes) in blood plasma, reported in milliosmoles per kilogram of water (mOsm/kg). It is normally set within a tight range by the body's thirst response and antidiuretic hormone (ADH). A lab can measure osmolality directly with a freezing-point-depression osmometer, but it can also be estimated from routine chemistry results using a well-established formula. This calculator performs that calculation.
The calculated osmolality formula
The standard formula sums the contribution of the three (or four) solutes that make up nearly all of plasma osmolality:
- Sodium is doubled (2 x Na) because each sodium ion is balanced by an accompanying anion, roughly doubling its osmotic contribution.
- Glucose (mg/dL) / 18 converts glucose to mmol/L using its molecular weight of about 180 g/mol.
- BUN (mg/dL) / 2.8 converts blood urea nitrogen to mmol/L using urea's nitrogen-based molecular weight.
- Ethanol (mg/dL) / 4.6 is added when a blood alcohol level is known, converting it to mmol/L using ethanol's molecular weight of about 46 g/mol.
Adding these terms gives calculated (or "estimated") serum osmolality in mOsm/kg.
Effective osmolality and the osmolar gap
Effective osmolality, also called tonicity, drops the urea and ethanol terms (2 x Na + glucose/18) because those two solutes move freely across cell membranes and do not pull water between compartments the way sodium and glucose do. Separately, when a lab has directly measured osmolality with an osmometer, subtracting the calculated value gives the osmolar gap. A gap that is much larger than normal suggests osmotically active substances in the blood that the formula does not account for, such as toxic alcohols (methanol, ethylene glycol, isopropanol) or mannitol.
When to consult a professional
This tool performs the standard arithmetic for education and quick reference; it is not a diagnosis. Any decision about fluids, electrolytes, or a suspected toxic ingestion should involve a licensed healthcare provider who can interpret the numbers alongside the full clinical picture.