Background: Coagulation is a treatment procedure for metalworking fluids (MWFs). This study aimed to optimize coagulation using four coagulants and compare the results. Methods: In this research, the outputs of a coagulation procedure in chemical oxygen demand (COD) removal, turbidity, and the release of oil were investigated using four coagulants, ferric chloride (FeCl 3 ), ferric sulfate (Fe 2 (SO 4 ) 3 , calcium chloride (CaCl 2 ), and aluminum sulfate (Al 2 (SO4) 3 . To optimize the process, a central composite design (CCD) and response surface methodology (RSM) were used. Results: Under optimal conditions for using calcium chloride (coagulant concentration 4.2 g/L and pH 3.71), COD and turbidity removals and oil released were respectively 93%, 96.9%, and 31.8 mL. COD and turbidity removals and oil released using aluminum sulfate as a coagulant under optimal conditions (5.36 g/L, pH 4.64) were 75.7%, 89.9%, and 28.9 mL, respectively. With respect to ferric sulfate under optimal conditions (6.25 g/L, pH 3.4), COD and turbidity removals were 60% and 84.1%, respectively and oil released was 16.1 ml. COD and turbidity removals and the greatest amount of oil released by ferric chloride under optimal conditions (3.16 g/L, pH 3.5) were 54.4%, 84.7%, and 15.9 ml, respectively.