Plasma Osmolality Calculator

About this calculator

Calculated serum osmolality estimates the concentration of dissolved particles in blood plasma from three routine lab values, using the standard formula: 2 × sodium (mEq/L) + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8. Sodium is doubled because it comes paired with an anion, while glucose and BUN are divided by their respective molecular-weight conversion factors to express them in the same mOsm/kg units as sodium.

Clinicians use this calculated value to assess hydration status and to compare against a directly measured osmolality — a large gap between the two, called the osmolar gap, can point to unmeasured substances in the blood such as toxic alcohols. It is also a routine part of evaluating conditions like diabetic ketoacidosis and severe dehydration, where osmolality readings guide fluid and electrolyte treatment.