Strongly bound excitons crucially affect the operation of organic optoelectronic devices. Nevertheless, precise experimental data on the exciton binding energy of organic semiconductors are lacking. In this study, we determine the exciton binding energy as the difference between the optical and transport bandgaps with a precision of 0.1 eV. In particular, electron affinities with a precision higher than 0.05 eV determined by low-energy inverse photoelectron spectroscopy allow us to determine the transport gap and the exciton binding energies with such high precision. Through a systematic comparison of a wide range of organic semiconductors, including 42 organic solar cell materials (15 nonfullerene acceptors, 4 fullerene acceptors, 13 low-bandgap polymers, 7 organic light-emitting diode materials, and 3 crystalline materials), we found that the exciton binding energy is one-quarter of the transport gap regardless of the materials. We interpret this unexpected relation from a hydrogen atom-like model, i.e., the quantized energy levels in a Coulomb potential between the positive and the negative charges.