A light detection and ranging (LiDAR) footprint is the spot area illuminated by a single laser beam, which varies with the beam direction and the regional terrain encountered. The geometric size of the LiDAR footprint is one of the most critical parameters of LiDAR point cloud data. It plays a very important role in the high-precision geometric and radiometric calibration of LiDAR systems. This letter utilizes space analytic geometry to derive LiDAR footprint equations and strictly considers laser beam attitude and terrain slope. Compared to the conventional plane geometry solution, the proposed approach is not only more rigorous in theory but also more powerful in practical applications.