The solvent flux across the membrane can be expressed as (where A is the area, Lp is the membrane permeability, σ is the reflection point, Δp is applied pressure and Δπ is the osmotic pressure)

The chemistry formula for osmotic pressure (π) is π = nRTV, where n represents the number of moles of solute particles, R is the ideal gas constant, T is the absolute temperature in kelvin, and V is the volume of the solution.