A laser eye surgery system includes a computer which scans a focused laser beam in a trajectory over a reticle or target and determines beam quality via laser light reflected from the target. The target may have a grid pattern of lines, with the diameter of the focused laser beam determined based on a time interval for the scanned beam to move onto a line of the grid pattern. Methods for measuring beam quality in a laser eye surgery system provide a direct, quantitative quality measurement of the focused laser beam, and may be performed quickly and automatically. Using scanning mirror position information together with signals resulting from laser light reflected from the target, the laser eye surgery system may also be calibrated.