The breaker index, defined as the ratio of wave height to local water depth for incipient breaking, is a key parameter in contemporary spectral wave models. In the present study, we modified the breaker index of Goda's formula using the SWAN model (Simulating WAves Nearshore). We suggested that the breaker index is a function of the deep-water wave steepness, bottom slope and relative water depth. The formula is evaluated against observational data, and the evaluation indicates that the modeled wave heights using the formula are significantly improved, especially for cases over mild and flat slopes. The formula is further evaluated through comparisons with published formulas and the improved formula overall shows better performance.