In this paper, we propose a hydroelastic design contour (HDC) that can be practically used for the preliminary design of pontoon-type rectangular very large floating structures (VLFSs). Using the design contour, we can easily predict the maximum bending moment of VLFSs in irregular waves. To develop the design contour, we first construct the hydroelastic response contours (HRCs) by extensively carrying out hydroelastic analyses considering various structural and wave conditions, namely, the bending stiffness and aspect ratio of VLFSs, incident wave length and angle, as well as the sea state. Based on the precalculated HRCs, we develop the HDC considering irregular waves. We then propose a preliminary design procedure for VLFSs using the HDC and demonstrate the design procedure for pontoon-type rectangular VLFSs. The HOC can significantly reduce time and effort for the design of VLFSs.