Enter the sodium concentration, glucose concentration, and blood urea nitrogen concentration into the calculator to determine the osmolarity.

Fluid Osmolarity Formula

The following formula is used to calculate the fluid osmolarity.

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


  • O is the osmolarity (mOsm/L) Na is the sodium concentration (mmol/L) Glucose is the glucose concentration (mg/dL) BUN is the blood urea nitrogen concentration (mg/dL)

To calculate the osmolarity, multiply the sodium concentration by 2. Then, divide the glucose concentration by 18 and the blood urea nitrogen concentration by 2.8. Add all these results together to get the osmolarity.

What is a Fluid Osmolarity?

Fluid osmolarity refers to the concentration of solute particles in a solution, or in this case, a bodily fluid. It is a measure of the body’s hydration status and can be used to diagnose and monitor conditions such as dehydration, kidney disease, or electrolyte imbalances. High fluid osmolarity indicates a high concentration of solutes, such as sodium, chloride, or glucose, and can suggest dehydration. Low fluid osmolarity, on the other hand, indicates a low concentration of solutes and can suggest overhydration.