Abstract: To satisfy the unceasingly increasing water demand and to palliate the scarcity of the easily mobilizable resources, the construction of dams was set as solution that should be taken into consideration to give rise to an impressive number of dams constructed all over the world. Among the different type of constructed dams, the earth dams are the most widespread and the technique of their construction has greatly progressed in time without elucidating all the phenomena and hence developing methods which agree with the dual level technical and economical. Besides the empirical methods are improved but remain with no absolute guarantee against destructive accidents. The infiltrations represent the main causes of damages pointed out on earth dams throughout the world for which increased attention is given to means to battle with their effects and it is always provided a water drainage system in the earth dams. However a drain cannot be effective unless it is well dimensioned and carefully laid down in the fill embankment. This is an impossible task to realize in so far as we make use of doubtful and succinct recommendation to fix the drain length. A graph is given in this study and allows reading directly the length of the internal drain blanket in a homogeneous earth dam. This was obtained after having carried out a simulation of a great number of earth dams by varying the total height, the embankment slopes and the material characteristics. For each dam, the drain length is changed within the acceptable limits of the hydraulic stability and the landslide stability of the down stream side embankment is calculated.