Calculate urine osmolality (osmolarity) from urine sodium, potassium, urea nitrogen and glucose, and compare it with the typical 50–1,200 mOsm/kg range.

An estimate from four solutes, not a measured lab osmolality. Educational use only.

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

Leave blank if urine glucose is negative (counts as 0).

Optional settings

Urine Osmolality 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 Urine Osmolality?

A urine osmolality test 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 Urine Osmolality?

The following steps outline how to calculate urine osmolality:

  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.