Currently, hexagonal image processing is mainly based on simulated data generated by square lattice conversions. For the evaluation of the conversion quality, this paper presents a method for ideal square to hexagonal lattice conversion. Based on the square lattice discrete-time Fourier transform (DTFT), the method first determines the values of the hexagonal discrete Fourier transform (HDFT), and performs the inverse HDFT to obtain the ideal conversion. This method provides a benchmark for evaluating other practical conversions and such evaluation is presented in this paper.