Difference Between Osmolality and Osmolarity

Osmolality vs Osmolarity

Osmolality and osmolarity are used to indicate the solute concentration of solute particles in a solution. The idea behind these two words is related to molarity and molality, but they have different meanings. In some cases molarity, molality and osmolality, osmolality can be similar values. For example, non-ionic solutes can be considered. But in the case of ionic solutes dissolved in a solvent, they have different values. To understand the two phenomena, we have to understand what these words mean. These two terms are widely used in reference to body fluids and also in biochemistry. Osmometers are used to measure these values.


Osmolality is a unit of concentration based on osmoles. Osmoles is a measurement of the solute particles in a solvent. Solutes may dissociate into two or more particles when it’s dissolved. A mole is a measurement of the solute, but osmoles is a measurement of these solute particles. Definition of osmolality is the osmoles of solute particles in a unit mass of the solvent (1 kg). So the unit of osmolality is Osm/kg. In clinics, milliosmoles is widely used, so the unit of osmolality can also be expressed as milliosmoles/ kg (mOsm/kg). For example, Serum osmolality is 282 – 295 mOsm/kg water. It is the same as molality where moles of solutes are measured in 1 kg of the solvent. The difference between molality and osmolality is the use of moles of solutes compared to osmoles of solutes respectively.


Osmolarity is the same as osmotic concentration. This is the measurement of solute concentration of a solution. The unit of osmolarity is Osm/L. It is defined as the number of osmoles of solute particles in one litre of solution. It can also be given as milliosmoles/liter (mOsm/L). For example, plasma and other body fluid osmolarity is 270 – 300 mOsm/L. Molarity is defined as the number of moles of solutes in a unit volume of a solution. In osmalolity, osmoles means, number of solute particles. For example, in a 1M sodium chloride solution, there is 1 moles of sodium chloride in 1 L. But when consider the osmolarity, there are 2 osmoles. This is because when sodium chloride is dissolved in a solution, sodium and chloride particles are considered as 2 separate solute particles, thus 2 osmoles. So for ionic compounds, the molarity and osmolarity will be different. But for non ionic molecules, since they don’t dissociate when dissolved, one mole of solute equals to 1 osmole. In disease diagnostics of patients, the difference between the calculated osmolarity and the measured osmolarity is considered, and this is known as the osmolar gap.

Osmolality vs Osmolarity

• The unit of osmolality is Osm/kg and the unit of osmolarity is Osm/L.

• In osmolality, number of solute osmoles in a unit mass of the solvent is considered, but in osmolarity, number of solute osmoles in a unit volume of solvent is considered.