This research proposes a scheme of area calculation system, focusing on image warping to top view. The sensor attached to camera is used to compensate the camera's orientation in real time. One constraint is that the calculated area is planar surface. In practice, the alignment of sensor and camera is imperfect. This error makes the calculated area size inaccurate. Therefore, calibration between camera and sensor is required by using Iterative Least Square method. Then, the extrinsic parameters derived from calibrated sensor and pre-computed intrinsic parameter will be used to generate a homography matrix. In top view image, we can directly count the number of target pixel. Finally, we will also find the relationship between rotation angle and size of each pixel in real-world unit with calculating percentage accuracy of this method. The experimental results show top view target area size generated from the tilted camera using information from the orientation sensor in real time.