A capsule for the preparation of a coffee extract from coffee contained therein and hot water injected under pressure in the capsule by a water injection device. The capsule has a frusto-conical body with a rim, sidewall, and an inlet wall and a lower delivery wall sealed to the rim and contains a predosed amount of coffee. The inlet wall may have a flat or convex portion and a structure in relief or in recess arranged for facilitating penetration of blades belonging to the injection device and extending on a substantially circular path of predetermined radius R1 equal to the radius R2 of the circular path along which the blades of the injection device extend.