Enter all but one of the sodium concentration, potassium concentration, blood urea nitrogen concentration, and blood glucose concentration into the calculator to determine the serum osmolality; this calculator can also evaluate any of the variables given the others are known.

Serum Osmolality Formula

The following formula is used to calculate the serum osmolality:

Osmolality = 2 times (Na + K) + (BUN / 2.8) + (Glucose / 18)


  • Osmolality is the serum osmolality
  • Na is the sodium concentration in mmol/L
  • K is the potassium concentration in mmol/L
  • BUN is the blood urea nitrogen concentration in mg/dL
  • Glucose is the blood glucose concentration in mg/dL

To calculate the serum osmolality, multiply the sum of sodium and potassium concentrations by 2. Then, add the blood urea nitrogen concentration divided by 2.8 and the blood glucose concentration divided by 18.

What is a Serum Osmolality?

Serum osmolality is a laboratory test that measures the concentration of all chemical particles found in the fluids of your blood. These particles include electrolytes such as sodium, chloride, and bicarbonate, as well as sugars like glucose, and waste products like urea. The test is used to evaluate the balance of water to solute in your body, which is crucial for vital functions such as maintaining blood pressure, controlling body temperature, and ensuring proper function of the muscles and nerves. Abnormal serum osmolality levels can indicate a variety of conditions including dehydration, kidney disease, diabetes, or alcohol intoxication. The test is typically performed by drawing a blood sample from a vein in your arm.

How to Calculate Serum Osmolality?

The following steps outline how to calculate the Serum Osmolality:

  1. First, determine the concentration of sodium ions (Na+) in the blood plasma (mEq/L).
  2. Next, determine the concentration of glucose in the blood plasma (mg/dL).
  3. Next, determine the concentration of blood urea nitrogen (BUN) in the blood plasma (mg/dL).
  4. Next, gather the formula from above = Serum Osmolality = 2(Na+) + glucose/18 + BUN/2.8.
  5. Finally, calculate the Serum Osmolality.
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Concentration of sodium ions (Na+) = 140 mEq/L

Concentration of glucose = 100 mg/dL

Concentration of blood urea nitrogen (BUN) = 20 mg/dL