AbstractResearch engineers use two types of energy equations to study single‐degree‐of‐freedom (SDOF) systems subject to earthquake induced ground motions. The first method uses anabsolute energyformulation; the second method uses arelative energyformulation. While the relative energy formulation has been used in the majority of previous investigations, this study shows that the absolute energy equation is physically more meaningful. For a given ductility ratio, the input energy demands calculated by both methods are significantly different for both the short and long period ranges although the results are similar in the intermediate period range. A comparison between the analytically predicted absolute input energy of a SDOF system with the experimentally measured input energy of a six‐storey braced steel frame shows good corr