Estimate urine osmolarity from urine sodium, potassium, glucose and urea nitrogen, or explore an algebraic inverse of the same model. The calculated value is in mOsm/L and is not a measured laboratory osmolality.

Estimates urine osmolarity (mOsm/L), a surrogate for laboratory osmolality. Educational use; inverse solves require a calculated value, not a measured laboratory result.

mg/dL N is nitrogen mass; mmol/L urea is the amount of urea.

Watch the full calculation explainer.

Osmolality Urine Formula

The following formula estimates urine osmolarity (mOsm/L), a surrogate for measured urine osmolality:

Osm = 2 × (Na + K) + (Glucose / 18) + (urine urea nitrogen (mg / dL) ​ / 2.8)

Variables:

  • Osm is estimated urine osmolarity (mOsm/L); Na and K are in mmol/L, while glucose and urine urea nitrogen are in mg/dL
  • Na is the concentration of sodium in urine
  • K is the concentration of potassium in urine
  • Glucose is the concentration of glucose in urine
  • UUN is urine urea nitrogen in mg/dL, not blood urea nitrogen; for urea in mmol/L, use that concentration directly as its osmolar contribution

To estimate urine osmolarity, double the sum of urine sodium and potassium in mmol/L, then add urine glucose in mg/dL divided by 18 and urine urea nitrogen in mg/dL divided by 2.8. Other solutes and incomplete salt dissociation may affect the estimate; algebraic inverses are model calculations, not substitutes for laboratory concentration measurements.

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. Measured results are expressed in milliosmoles per kilogram of water (mOsm/kg). Values vary with fluid intake and clinical context; a clinician should interpret the laboratory result. The calculator instead estimates osmolarity in mOsm/L and does not diagnose a condition.

How to Calculate Osmolality Urine?

The following steps outline how to calculate the Osmolality Urine:

  1. First, collect a urine sample.
  2. For the calculated estimate, obtain laboratory concentrations of urine sodium, potassium, glucose and urea nitrogen from the same sample.
  3. Record sodium and potassium in mmol/L (or mEq/L), and glucose and urine urea nitrogen in mg/dL; use the calculator’s explicit unit selections when values differ.
  4. Calculate estimated urine osmolarity using the four-solute equation above.
  5. Keep a measured osmometer result separate from this estimate; discuss clinical interpretation with a qualified clinician.

Example Problem:

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

Example: urine Na = 50 mmol/L, K = 25 mmol/L, glucose = 180 mg/dL and urea nitrogen = 280 mg/dL give 2 × (50 + 25) + 180/18 + 280/2.8 = 260 mOsm/L estimated osmolarity.