The fundamental understanding of sodium storage mechanisms in amorphous carbon is essential to develop high-performance anode materials for sodium-ion batteries. However, the intrinsic relation between the structure of amorphous carbon and Na+ storage remains to be debated due to the difficulty in controlling and characterizing the local atomic configurations of amorphous carbon. Here we report quantitative measurements of Na+ storage in a low-temperature dealloyed hard carbon with a tunable local structure from completely disordered micropores to gradually increased graphitic order domains. The structure-capacity-potential correlation not only verifies the disputing "adsorption-intercalation"mechanisms, i.e., Na+ intercalation into local graphitic domains for the low-voltage plateaus and adsorption in fully disordered carbon for the sloping voltage profiles, but also unveils a new mechanism of Na+ adsorption on defective sites of graphitic carbon in the medium-potential sloping region. The quantitative investigations provide essential insights into the reaction mechanisms of Na+ with amorphous carbon for designing advanced sodium-ion battery anodes.