Plasma Osmolality Calculator

How it is calculated

The tool estimates serum osmolality from three routine lab values: osmolality = 2 × Na + glucose ÷ 18 + BUN ÷ 2.8. Sodium is in mmol/L (the same number as mEq/L for sodium), glucose and BUN (blood urea nitrogen) are in mg/dL, and the result is in mOsm/kg, shown to one decimal. Sodium is doubled to account for its paired anions, and the divisors 18 and 2.8 convert mg/dL of glucose and urea nitrogen into mOsm. Values in mmol/L for glucose or urea will give a wrong answer, because the formula expects mg/dL. Inputs outside plausible limits show no result: sodium must be 50 to 250, glucose 0 to 5000 and BUN 0 to 500. It gives the calculated figure only; it does not compare it with a measured osmolality or interpret it.

Common mistakes

• Entering glucose or urea in mmol/L when the fields need mg/dL. A glucose of 7 mmol/L typed as 7 gives a glucose term of 0.39 instead of about 7, and the result (286.4) is too low.
• Typing urea where BUN is needed, or the other way round. The two are not the same number, so use the value your lab labels as BUN in mg/dL.
• Entering sodium in the wrong form, such as 1.38 or 13.8, or leaving a field empty. Sodium below 50 or any empty box gives no result.
• Treating the output as a measured osmolality. It is a calculated estimate, and a large difference from a measured value (the osmolar gap) is a separate step this tool does not do.
• Entering negative numbers or values far outside the body's range, such as glucose above 5000. The tool shows no result rather than a figure, so check for a typo.