Osmolality vs Osmolarity

When navigating the world of hydration or re-hydration products, one confusing attribute of such products comes up a lot, osmolality or osmolarity? Fortunately, here at the The Osmolality Lab, we clear up the confusion.

These two terms are not really that different. They both represent the osmotic pressure of the specific concentration of solutes in a solution but expressed in two different ways with two slightly different units. Osmolality refers to the osmotic pressure of a solution due to its solutes as measured in osmoles (Osm) or milliosmoles (mOsm) but is expressed per unit weight (kilograms), i.e., Osm/kg or mOsm/kg. Osmolarity, on the other hand, expresses the same property but per unit volume (liters), i.e., Osm/l or mOsm/l. Now, 1 kilogram of pure water has a volume of exactly 1 liter so for pure water the two are effectively the same. However, as the density of the solution changes, which happens when solids dissolve into a solution, then the osmotic pressure can be a little different if expressed per kilogram or per liter, and especially at higher concentrations of dissolved solids where density changes are bigger.

Since most hydration beverages are precisely that, a liquid beverage, or a solid that is dissolved into a liquid, then it would make most sense to report osmolarity right? But when measuring osmotic pressure (osmometry) it is practically easier to weigh out small samples rather than use a specific small volume, thus, osmolality is typically reported. In terms of the hydration market, expressing osmotic pressure in terms of osmolality is wholly acceptable but one can convert from one to the other if the density of the beverage is accurately known.


Interested in learning more about hydration and osmolality? Check our Hydration Education page here.