Serum Osmolality Calculator

How it is calculated

The engine uses the common estimating formula osmolality = 2 × Na + glucose / 18 + BUN / 2.8. Sodium (Na) is entered in mmol/L (the same number as mEq/L), glucose in mg/dL and blood urea nitrogen (BUN) in mg/dL. The sodium term is doubled to account for its accompanying anions; dividing glucose by 18 and BUN by 2.8 converts mg/dL into mmol/L. The sum is rounded to a whole number and shown in mOsm/kg. No ethanol or other term is included, and no reference range is applied, so the figure is an estimate to compare with a lab-measured osmolality, not a diagnosis. All three fields are required.

Common mistakes

• Entering glucose in mmol/L (for example 5.5) instead of mg/dL (about 100); the glucose term then shrinks to almost nothing and the result is too low.
• Entering BUN as urea in mmol/L, or as urea rather than urea nitrogen; the 2.8 divisor only fits BUN in mg/dL.
• Entering sodium in mg/dL or doubling it before typing it in; the engine already multiplies sodium by 2.
• Comparing the calculated value with a measured osmolality that was taken at a different time from the blood chemistry; the gap between them is only meaningful when both come from the same sample.
• Expecting alcohol to be included: the formula has no ethanol term, so alcohol raises the measured value but not this calculated one.