Osmotic pressure is the pressure that would need to be applied to prohibit a pure solvent from passing into a given solution by osmosis. The osmotic pressure of a solution depends on the concentration of dissolved solute particles and can be calculated by the modified van’t Hoff equation:

Note: depends on solute type and concentration solution and can be found for NaCl solutions in references [1] and [2]. is the ideal gas constant (L bar K^{-1} mol ^{-1}). The OMD used the OLI activity coefficient model (OLI Systems, Morris Plains, NJ) to calculate the osmotic pressure of NaCl concentrations in studies initially uploaded to the database.

- [1] Pitzer, K. S., Peiper, J. C., & Busey, R. H. (1984). Thermodynamic properties of aqueous sodium chloride solutions. Journal of Physical and Chemical Reference Data, 13(1), 1-102.
- [2] Hamer, W.J. and Wu, Y. (1972). Osmotic coefficients and mean activity coefficients of uni-univalent electrolytes in water at 25 °C. Journal of Physical and Chemical Reference Data 1, 1047–1100.