Serum osmolality calculator
Calculated osmolality is an estimate of how concentrated your blood is, worked out from your sodium, glucose and BUN results rather than measured directly. Type your results in the units on your report and it works out Calculated Osmolality as you type, in your browser.
AI-verified against cited sources. Awaiting expert reviewLast reviewed 2026-10-03Electrolytes & Minerals
Type your results above, or fill in the example, to see Calculated Osmolality.
Nothing you type here leaves your browser.
Educational information, not medical advice. Blood Test Tracker doesn’t diagnose or treat, and it doesn’t replace your clinician. Talk with your clinician before changing medications, supplements or care.
How it’s worked out
The formula
Osmolality (mOsm/kg) = 2 × Na (mmol/L) + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8. In SI units: 2 × Na + glucose + urea, all in mmol/L. Some analyzers use 1.86 × Na + glucose ÷ 18 + BUN ÷ 2.8 + 9, and some add blood alcohol (mg/dL) ÷ 5.
A worked example
Example numbers, not anyone’s real results, run through the same code as the calculator above.
| Sodium | 140 mmol/L |
|---|---|
| Glucose | 90 mg/dL |
| BUN | 14 mg/dL |
| Calculated Osmolality | 290 mOsm/kg |
For an adult, this result is in the usual range.
What Calculated Osmolality tells you
Calculated osmolality comes from a formula, not a separate lab test. It adds up the main dissolved particles in your blood. Sodium counts twice to include its partner ions, and glucose and urea nitrogen (BUN) add smaller amounts. In common US units the formula is 2 × sodium + glucose ÷ 18 + BUN ÷ 2.8. Labs and analyzers use slightly different versions, so the same blood can give slightly different numbers. A measured osmolality test, run on the blood itself, is a separate marker.
The calculated value gives a quick estimate of blood concentration from a routine chemistry panel. Its main use is comparison. If the measured osmolality is 10 or more higher than the calculated value, something besides sodium, glucose and urea, such as an alcohol, may be in the blood. That difference is called the osmolal gap. On its own, the calculated value mostly follows sodium and glucose, so it is read together with those results and your symptoms.
Read the full page on Calculated Osmolality, with what a high or low result can mean.
Usual ranges
The ranges the result above is placed on. Your own lab’s printed range comes first if it gives one.
Sources
The equation
- Water and Sodium Balance, Merck Manual Professional Version
- Metabolic Acidosis, Merck Manual Professional Version (2025)
- Laboratory Data: Standard Biochemistry Profile (BIOPRO_L), NHANES August 2021-August 2023, CDC National Center for Health Statistics (2025)
4 more sources for the ranges and the explanation
- Osmolality, Serum (test ID UOSMS), Mayo Clinic Laboratories (catalog copy hosted by Schneck Medical Center)
- Osmolality (test 002071), Labcorp
- Osmolality, Serum or Plasma (test 0020046), ARUP Laboratories
- Diagnostic Laboratories: Critical Values, Houston Methodist
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. The calculator is version 1.0.0 of our code for this equation, tested against published examples. How we build and check these pages.