A generic method is proposed to use the flammability diagram to determine the flammable state of a mixture and the dilution requirements. All graphs are derived from theory, without any empirical constants. This method is comprehensive and consistent with a same treatment for fuel(s)/oxidizer(s)/diluent(s). The theoretical LFL lines are generally lower than the experimental lines due to the assumption on constant flame temperature. So, the prediction is usually conservative, which is good for estimation purposes. © 2013 American Institute of Chemical Engineers Process Saf Prog 33: 70–76, 2014