Osmolality, in biology and chemistry, is a measure of moles of solute per kg of water. (Osmolarity is similar but measures moles per litre of solvent.) Osmolality increases with dehydration and decreases with overhydration.

In normal people, increased osmolality in the blood will stimulate secretion of ADH (antidiuretic hormone). This will result in increased water reabsorption, more concentrated urine, and less concentrated plasma. A low serum osmolality will suppress the release of ADH, resulting in decreased water reabsorption and more concentrated plasma.

Normal osmolality in blood plasma is about 280 - 296 mosm/kg. This is contributed to mostly by sodium, chloride, potassium, urea and glucose, as well as other ions and substances in the blood.

An increase in any osmotically active compound or ion in the blood will cause an increase in the osmolality.

Relationship to molality

Osmolality is molality times number of particles per molecule times another factor.

Units of measurement

SI units: moles per kilogram

The osmole is a non-SI unit, equal to the number of moles of osmotically active particles in an ideal solution. So a mole of glucose equals an osmole of glucose, whereas a mole of sodium chloride equals 2 osmoles and a mole of calcium dichloride equals 3 osmoles due to dissociation.

Measurement of osmolality

Osmolality can be measured with an osmometer by determining the fall in freezing point when compared to deionised water (which has an osmolality of 0 mosm/L). It can also be estimated from levels of specific solutes measured in the blood.

If there is a difference between the measured and estimated osmolalities, this may be because of another osmotically-active substance that was not accounted for.