The Concentration of Your Blood

Osmolarity measures solute particles per liter of serum — the number that guides intravenous fluids, hydration assessment, and electrolyte management. This osmolarity calculator applies the standard clinical formula to your lab values.

The Formula

Osmolarity ≈ 2 × Na⁺ + glucose/18 + BUN/2.8. Sodium is doubled because its counter-ions contribute equally; glucose and BUN are divided to convert mg/dL into mmol/L. The result lands in the normal band of 275–295 mOsm/L for healthy patients.

The Osmolar Gap

Enter a measured osmolarity and the calculator reports the gap. A gap above ~10 mOsm/L signals unmeasured osmotically active substances — the classic screening clue for methanol, ethylene glycol, or other alcohol poisonings.