In this paper a technique for stereo camera calibration from a known planar calibration object is proposed. Both the intrinsic parameters and the relative orientation are calculated. The proposed algorithm uses the two homogeneous linear constraints on the image of the absolute conic arising from the plane homographies for each camera and position. In addition to these, linear constraints on the relation between the images of the absolute conic, for the two cameras in the stereo pair, are calculated. The derivation of these constraints is made possible by considering the rigidity of the stereo pair. Using all these constraints simultaneously in a linear system gives a stereo calibration technique that is less sensitive to noise, compared to just using the single camera constraints. As a part of the algorithm, a simple way to calculate the infinite homography from two stereo views of a plane is presented. The performance of the algorithm is shown in experiments on both simulated and real data.