As the volume of a droplet decreases on a solid surface, there is an initial stage during which the contact area is constant followed by a stage during which the contact angle depends on the droplet size. At the present time, it is very difficult to predict this kind of process theoretically by considering the effect of the surface energy density of the solid. In the present study, we show that variations in geometrical parameters (contact area radius, droplet height, and contact angle) of water droplets ranging from micro- to millimeters in size during volume reduction can be predicted on the basis of a wettability model that considers the surface energy density of the liquid and the critical surface energy of the solid. The fundamental wetting behavior of a droplet under volume reduction process is observed by focusing on natural evaporation of the droplet and is discussed by quantifying the surface properties of both the solid and the liquid.