In osmotically driven membrane processes (ODMPs), such as forward osmosis (FO), the concentration of the draw solute and the related osmotic pressure play a critical role in mass transport and overall process performance. Search of the literature reveals that the concentration units used to describe draw solutes vary and the methods for deriving osmotic pressure from those concentrations are often unclear or not discussed. This paper recommends the use of molality and identifies the benefit of experimentally determined vant Hoff indices when calculating osmotic pressures.