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We theoretically and experimentally present a novel method to accurately extract coupling coefficients of directional couplers using a single circuit. It utilizes the phenomenon of resonance splitting induced by mode coupling in a ring resonator, where the splitting distance is directly proportional to the coupling coefficient of the coupler that couples two modes. Compared with previous approaches, it doesn't rely upon measured amplitudes for parameter extraction, therefore, this method is insensitive to the uncertainties in the losses of other components in the circuit such as fiber/chip couplers, waveguides and resonators, as well as gage errors. More importantly, it can work with very weak coupling coefficients, down to 10^−4.