Aiming at existing methods unable to reveal the definition of dilution in welding accurately, such as the limited formula’s range of 0~1 mathematically, a new way which can be widely applied to various welds alloying either one or more compositions was proposed. Dilution values obtained using different methods through chemical composition analysis were compared and the effect of heat inputs on dilution was researched. The results show that the proposed method fabricated mathematically agrees well with the definition of dilution in welding. Especially, by dilution increasing to a level higher than the critical value, it reveals the fact that a weld can be concentrated as well when the filler wire is rich in the same composition as the substrate does. Besides, the method demonstrates that the gross dilution for a bead is a combined result that is affected by each individual composition. However, whether a composition is diluted or concentrated, composition’s concentrations are propelled by the increase of heat input toward that of the original substrate. During the process, the dilution value changes sharply before the heat input reaches the transition point (TP) of 0.18 kJ/mm, while once the heat input is beyond TP, the dilution becomes stable.