Osmotic potential (ψs) is commonly measured by either thermocouple psychrometry or hygrometry after previously freezing and thawing the sample to break cell membranes and reduce ψp to zero; normally a correction is made for the dilution by apoplastic water.Osmotic pressure is directly proportional to the number of solute atoms or molecules; ions exert more pressure per unit mass than do non- electrolytes. Electrolyte ions require facilitated diffusion and active transport to cross the semi-permeable membranes. Osmosis is also impacted by how the solute behaves in water, which is where Van't Hoff's factor comes in. Basically, the Van't Hoff's factor of a solute is determined by whether or not a solute stays together or breaks apart in water. Some solutes break apart and form ions, or charged atoms, in water. As shown here, table salt, NaCl, will form sodium (Na+) and Chlorine (Cl-) ions in water. The Van't Hoff's factor of NaCl is two because it breaks into two ions.

When placed in water, some molecules such as sucrose stay together and do not form ions. Since sucrose does not break into ions, it has a Van't Hoff's factor of 1.

Osmotic Potential = iMRT where i = Van Hoff's factor, M= Molar concentration of solution (Mol/L), R= Ideal gas constant (0.08206 L atom mol-1 K-1), T = temperature in Kelvin (K). VanHoff is the factor is the measure of the number of ions a solute will form when dissolved in water.