Plasma Osmolality Calculator

Plasma osmolality is the concentration of all dissolved particles in blood. It reflects the balance between water and solutes, and the body defends it tightly between 275 and 295 mOsm/kg through thirst, vasopressin release and the kidney.

The formula

US conventional units, with glucose and urea in mg/dL:

Posm = 2 × Na + (glucose / 18) + (BUN / 2.8)

SI units, with glucose and urea in mmol/L, where the divisors disappear because millimoles per liter of a non-dissociating solute already are milliosmoles:

Posm = 2 × Na + glucose + urea

Mixing the two is the classic error. A glucose of 5 mmol/L put through the mg/dL formula contributes 0.3 instead of 5, which is why this calculator asks which units your lab report uses.

Sodium is doubled because it travels with a roughly equal concentration of anions, mostly chloride and bicarbonate, to keep the blood electrically neutral.

Reading the number

For hyponatremia, how fast the level fell matters more than where it landed. A slow drift to 265 is tolerated far better than a rapid fall to the same number, and correcting too quickly carries its own serious risk.

The osmolal gap, which is the point of calculating this at all

Labs can measure osmolality directly by freezing-point depression. The gap between what they measure and what this formula calculates tells you whether something is in the blood that the formula does not know about:

Osmolal gap = measured − calculated

A gap between roughly −10 and +10 is normal. Above 10 means unmeasured osmoles: methanol, ethylene glycol, isopropanol, ethanol, mannitol, or glycine from surgical irrigation fluid. In an unexplained metabolic acidosis or an altered mental state of unknown cause, this is one of the more useful numbers available, and it is available within minutes.

Ethanol is the usual innocent explanation. Roughly every 100 mg/dL of blood alcohol adds about 22 mOsm/kg, so a gap fully accounted for by a known alcohol level makes the toxic alcohols much less likely.

Enter a measured value in the optional field and the calculator works the gap out.

Corrected sodium

High glucose pulls water out of cells and into the blood, diluting the sodium that is already there. A sodium of 130 alongside a glucose of 600 mg/dL is usually not true hyponatremia at all. The conventional correction adds 1.6 mEq/L to the measured sodium for every 100 mg/dL of glucose above normal, and this calculator applies it automatically whenever glucose is above 200.

Osmolality is not tonicity

Urea crosses cell membranes freely. It raises measured osmolality without pulling any water across those membranes, so it does not shrink or swell cells. Effective osmolality, or tonicity, leaves it out entirely: 2 × Na + glucose. When the question is whether brain cells are swelling, tonicity is the number that answers it, and osmolality is not.

Educational use only. This calculation supports clinical judgment; it does not substitute for it.