ABSTRACT The analytical solutions describe the behavior of physical phenomenon in nature. Capillary force is one of them and it acts in different manners. Particularly, the rise of a fluid through a capillary path is a phenomenon constantly present in nature. The document presents an analytical solution to describe the rate of rise of a wetting phase (displacing fluid) through a capillary tube in presence of a non-wetting phase (displaced fluid), taking into consideration the viscosity term and tube length effect. The Lucas-Washburn equation (1921) predicts the fluid rise in function of time. The model is accurate in late times and in early times or transient conditions the model has a variation compared with experimental data due to fluid inertia, which is not considered.