What Is the Serum Osmolality Calculator?
Serum osmolality measures the concentration of solutes dissolved in blood plasma, expressed in milliosmoles per kilogram (mOsm/kg). This calculator estimates calculated osmolality from common lab values. It uses US conventional units (mg/dL for glucose, BUN, and ethanol; mEq/L for sodium). The normal reference range is roughly 275–295 mOsm/kg.
How to Use It
Enter the patient's serum sodium, glucose, and blood urea nitrogen (BUN). If an ethanol level is available, enter it as well; otherwise leave it at zero. The calculator returns the estimated osmolality. Comparing this calculated value with a measured osmolality gives the osmolar gap, which can flag the presence of unmeasured osmoles such as methanol or ethylene glycol.
The Formula Explained
The equation is $$\text{Osm} = 2 \times \text{Na} + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8} + \frac{\text{Ethanol}}{3.7}$$. Sodium is doubled to account for accompanying anions (mainly chloride and bicarbonate). Glucose is divided by 18 and BUN by 2.8 to convert mg/dL into mmol/L (their molecular weights). Ethanol is divided by 3.7 to convert its contribution into mOsm/kg.
Worked Example
For Na = 140, Glucose = 90, BUN = 14, Ethanol = 0:
$$2 \times 140 = 280; \quad 90 / 18 = 5; \quad 14 / 2.8 = 5; \quad 0 / 3.7 = 0.$$ Total = $$280 + 5 + 5 + 0 = 290 \text{ mOsm/kg},$$ which is within the normal range.
FAQ
What is the osmolar gap? It is the measured osmolality minus this calculated value. A gap greater than about 10 mOsm/kg suggests unmeasured solutes.
Why include ethanol? Ethanol is osmotically active and a common cause of a raised osmolar gap, so adding it improves accuracy in intoxicated patients.
Is this a diagnostic tool? No. It is an educational estimate and should not replace clinical judgment or laboratory measurement.
