A novel tool for the establishment of cardiothoracic ratio in chest radiographs is explained in this paper. Cardiothoracic ratio is the vital parameter in the evaluation of heart size. Identification of patients having enlarged heart (cardiomegaly) or narrow heart can be determined through this simple tool. Medical chest x-ray images are taken for consideration and it is been undergone through the following operations like pre processing, feature extraction and computation of cardiothoracic ratio .In the feature extraction process using zernike moment algorithm the heart is extracted. Based on cardio thoracic ratio values the various heart sizes are identified and tabulated. More than 1200 normal chest x-rays are tested and compared with theoretical values. Also comparison is shown for various sample sizes. Matlab 2012R simulation tool is used for this process. This method will help the physicians even in identifying one of the heart disease namely cardiomegaly (enlarged) which may leads to heart failure. © 2018, Institute of Advanced Scientific Research, Inc. All right reserved.