The formula for water potential is Ψ = Ψs + Ψp, where Ψ is the water potential, Ψs is the solute potential, and Ψp is the pressure potential. Solute potential is the measure of the effect of solutes on the water potential, while pressure potential is the measure of the effect of pressure on the water potential. The water potential formula is used to determine the direction of water movement in plants and other organisms.