Enter the Measured Osmolality, sodium concentration, glucose concentration, and Blood Urea Nitrogen level into the calculator to determine the Osmolal Gap.

Osmolal Gap Formula

The following formula is used to calculate the Osmolal Gap.

OG = Measured Osmolality - Calculated Osmolality


  • OG is the Osmolal Gap Measured Osmolality is the osmolality measured in a laboratory Calculated Osmolality is calculated using the formula: 2 * (Na+) + (Glucose/18) + (BUN/2.8)


  • Na+ is the concentration of sodium in the blood Glucose is the concentration of glucose in the blood BUN is the Blood Urea Nitrogen level

To calculate the Osmolal Gap, first calculate the Calculated Osmolality by multiplying the sodium concentration by 2, adding the glucose concentration divided by 18, and the BUN divided by 2.8. Then subtract this value from the Measured Osmolality.

What is an Osmolal Gap?

The Osmolal Gap is a diagnostic tool used in medicine to identify the presence of certain toxins in the body. It is the difference between the measured osmolality (the concentration of solutes in the blood) and the calculated osmolality. A higher than normal Osmolal Gap can indicate the presence of toxins such as methanol, ethylene glycol, or isopropyl alcohol in the blood.

How to Calculate Osmolal Gap?

The following steps outline how to calculate the Osmolal Gap:

  1. First, determine the Measured Osmolality (osmolality measured in a laboratory).
  2. Next, determine the concentration of sodium in the blood (Na+).
  3. Next, determine the concentration of glucose in the blood (Glucose).
  4. Next, determine the Blood Urea Nitrogen level (BUN).
  5. Using the formula: Calculated Osmolality = 2 * (Na+) + (Glucose/18) + (BUN/2.8), calculate the Calculated Osmolality.
  6. Finally, calculate the Osmolal Gap using the formula: OG = Measured Osmolality – Calculated Osmolality.
  7. 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.

Measured Osmolality = 300

Na+ = 140

Glucose = 100

BUN = 20