Conventional measurement methods for large flat mirrors are generally difficult and expensive. In most cases, comparison with a master or a reference flat similar in size is required. Using gravity, as in modern pendulum-type electronic levels, takes advantage of a free reference to precisely measure inclination or surface slopes. We describe using two electronic levels to measure flatness of large mirrors. We provide measurement results on a 1.6-m-diameter flat mirror to an accuracy of 50 nm rms of low-order Zernike aberrations.