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

Osmolality Urine Formula

The following formula is used to calculate the osmolality of urine:

Osm = 2 * (Na + K) + (Glucose / 18) + (BUN / 2.8)


  • Osmolality is the osmolality of urine
  • Na is the concentration of sodium in urine
  • K is the concentration of potassium in urine
  • Glucose is the concentration of glucose in urine
  • BUN is the concentration of blood urea nitrogen in urine

To calculate the osmolality of urine, multiply the concentration of sodium in urine by 2, add the concentration of potassium in urine, divide the concentration of glucose in urine by 18, divide the concentration of blood urea nitrogen in urine by 2.8, and sum all the results.

What is an Osmolality Urine?

Osmolality urine is a medical test that measures the concentration of solutes in the urine. Solutes are substances that are dissolved in a liquid, in this case, the liquid is urine. The test is used to evaluate the kidney’s ability to concentrate or dilute urine, which is a key function of the kidneys. It can help diagnose various conditions that can affect the balance of fluids in the body, such as diabetes insipidus, dehydration, or overhydration. The test is usually performed by collecting a urine sample in a clean container, which is then sent to a laboratory for analysis. The results are expressed in milliosmoles per kilogram (mOsm/kg) and the normal range can vary depending on the laboratory, but it is typically between 500 to 800 mOsm/kg in adults.

How to Calculate Osmolality Urine?

The following steps outline how to calculate the Osmolality Urine:

  1. First, collect a urine sample.
  2. Next, measure the osmolality of the urine sample using an osmometer.
  3. Next, record the osmolality measurement in milliosmoles per kilogram (mOsm/kg).
  4. Finally, calculate the Osmolality Urine.
  5. After obtaining the osmolality measurement, check the reference range for normal urine osmolality to determine if the result is within the expected range.

Example Problem:

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

osmolality of urine (mOsm/kg) = 500