为表示某溶液的渗透压，有时使用与该溶液具有等渗的非电解质溶液的克分子浓度，称此为克分子渗透压浓度，以渗透克分子（osmole，Osm），或毫渗透克分子（m osmole， m Osm）来表示。因为1个重量克分子的理想的非电解质溶液的冰点下降度（△）相当于1.858℃，所以测定出某溶液的冰点下降度，就可由此换算出其克分子渗透压浓度。例如，1%的食盐水（171毫摩尔／千克）的冰点下降度为0.595℃，故克分子渗透压浓度相当于320m Osm。克分子渗透压浓度与容量克分子浓度（molarity）和重量克分子浓度（molality）相对应，被区别为容量渗透浓度（osmolarity和Osm／1）和重量渗透浓度（Osmolality，Osm／kg）。

Osmolarity

Osmolarity is a measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane (one which allows free passage of water and completely prevents movement of solute) compared to pure water. Osmolarity is dependent on the number of particles in solution but independent of the nature of the particles. For example, 1 mole of glucose dissolved in 1 litre of water has an osmolarity of 1 osmole (osm) /l. If 1 mole of another sugar, such as sucrose were added to the same litre of water, the osmolarity would be 2 osm/l. It doesn't matter that the solution contains 1 mole of glucose and 1 mole of sucrose. If 1 mole of NaCl were dissolved in 1 litre of water it would produce a 1 mol/l NaCl solution with an osmolarity 2 osm/l because NaCl dissociates into Na⁺ and Cl^- (two particles) in solution. This is true of all compounds that dissociate in solution. Na₂SO₄, which dissociates into Na⁺, Na⁺ and SO₄^2-, to give 3 particles per molecule produces 3osm/l for every mole dissolved in 1 litre.

If two solutions contain the same number of particles they may be said to be iso-osmotic (isosmotic) with respect to each other. If one solution has a greater osmolarity than another solution it is hyperosmotic with respect to the weaker solution. If one solution has a lower osmolarity than another solution then it is hypo-osmotic (hyposmotic) with respect to the stronger solution. Iso, hyper and hypo osmolarity should always be stated with respect to another solution. For example, a 1 mol/l NaCl solution is hyperosmotic with respect to 1 mol/l glucose solution.