There have been quite a few types of equation of state and abundant pi-a curves of various lipid monolayers at the air-water or the oil-water interface in the literature. However, it has been a problem to interpret mechanochemical properties of bilayer vesicles from the pi-a information of the monolayer. In fact, even the bilayer surface pressure has not yet been well characterised although the monolayer surface pressure has already been traditionally defined as the lowering in the surface tension from the clean interfacial tension due to the presence of the monolayer. The monolayer-bilayer correspondence problem therefore could not be well defined and completely solved in spite of its importance in practice to apply the monolayer pi-a data to elucidate bilayer vesicle properties and to theoretically study biomembranes. In the present analysis, we thus firstly define the bilayer leaflet pressure as the intrinsic pressure of the lipid layer system, which is the sum of the net repulsive interaction of the lipids within the lipid layer and the hydrophilic interaction of the headgroups.