This paper studies a log likelihood ratio (LLR) calculation method for an on-off keying symbol in screen to camera communication (SCC) systems. To enhance the reliability of SCC, forward error correction is an essencial technique. For soft decision decoding of capacity approaching codes such as turbo code and low density parity check code, it is necessary to calculate the LLR as an input of the decoder. However, it has not been examined in any detail in the conventional studies on the SCC systems. This motivates us to focus on the LLR calculation method to realize a more reliable SCC system. For the calculation of LLR, we estimate the conditional probability of a received signal intensity assuming both Gaussian and Rayleigh distributions. To evaluate the coding scheme coupled with the proposed LLR calculation, we conduct an experiment using actual commercial devices. The results show that the proposed LLR calculation appropriately works and reduces bit errors.