What this calculator does
Calculated serum osmolality works out estimates blood osmolality from a basic metabolic panel, plus the osmolal gap. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaOsmolality (mOsm/kg) = 2 × Na + Glucose/18 + BUN/2.8 (+ Ethanol/3.7 if measured)
The inputs explained
| Field | What to enter |
|---|---|
| Sodium (mEq/L) | A number, measured in mEq/L. Starts at 140. |
| Glucose (mg/dL) | A number, measured in mg/dL. Starts at 90. |
| Blood urea nitrogen (BUN) (mg/dL) | A number, measured in mg/dL. Starts at 14. |
| Ethanol (0 if not measured) (mg/dL) | A number, measured in mg/dL. Starts at 0. |
| Lab-measured osmolality (0 if unknown) (mOsm/kg) | A number, measured in mOsm/kg. Starts at 0. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with sodium
Every other input is held at the calculator’s starting values while sodium varies. Select any row to load that scenario into the calculator.