Humanoids are, by definition, robotic systems for which the control of interaction forces with the environment is elemental for the accomplishment of any loco-manipulation task. Any feedback controller of the interaction forces would invariably require some form of measure or estimation of the forces actually exchanged between the robot and the environment. This paper describes a method for estimating the contact forces for humanoids physically interacting with the environment. While analogous problems have been solved in the case of serial chain manipulators, the same study is missing for humanoids. With respect to the estimation problem for manipulators, these robotic systems introduce some peculiar challenges related to the presence of redundantly actuated kinematic loops. The considered interaction is not limited to specific points of the humanoid body since the proposed method also allows localization of the contact point without resorting to other sensing techniques, like, e.g., vision. The method has been extensively tested on the humanoid NAO.