Extensive studies on the discharge coefficients of open-type swirl injectors, which have a swirl chamber diameter equal to their nozzle diameter, have been conducted. Nineteen open-type swirl injectors with a broad range of swirl injector geometric constants from 2.86 to 18.13 were designed and manufactured. When varying the injection pressure drop from approximately 8 bar to 12 bar, static pressure and flow rate data were collected and analyzed to calculate the discharge coefficient. The measured discharge coefficients were confirmed to be precisely expressed as a function of the swirl injector geometric constant. Previous empirical equations, respectively induced from closed-type swirl injectors and open-type swirl injectors, were compared with the present experimental results to examine their accuracy. Though previous equations from open-type swirl injectors were better than those from closed-type swirl injectors, most of the equations could not satisfactorily predict the discharge coefficients from the present study. Two novel equations for the discharge coefficient of open-type swirl injectors are suggested: one was empirically obtained through a linear regression analysis of the present test data and the other was derived by modifying the design method of the closed-type swirl injector. It was found that both the modified design method and the present empirical equation matched well with the experimental data. Another equation, which can be used both for closed-type and open-type swirl injectors, is also proposed from a non-linear regression analysis using previous and present experimental data.