The Hawaiian Ridge provides a 50 million year record of the interaction between a plume of hot rock rising through the mantle and the westward motion of the Pacific plate. One feature related to the plume-lithosphere interaction, known as the 'swell', is a broad region of elevated bathymetry dynamically supported by the thermal buoyancy of plume material accumulating beneath the lithosphere. Prior studies have examined changes in swell dimensions to estimate fluctuations in the rate at which hot mantle material flows through the Hawaiian plume (buoyancy flux) to the base of the lithosphere. To improve upon these estimates, we developed a method to constrain fluctuations in Hawaiian plume buoyancy flux from swell size, using a model of the deforming plume head that assumes non-Newtonian rheology, while accounting for changes in the velocity of the Pacific plate and subsidence of the swell attributed to heat loss. To analyze the isolated signal of the swell we sampled cross sections of the swell from modern bathymetric datasets, corrected for ocean sediment thicknesses and lithospheric subsidence. By comparing these observations with model predictions of swell shape, we constrain the plume buoyancy flux over time. Our results show that the buoyancy flux of the Hawaiian plume has more than doubled between ~50 Ma and the present, and suggest that these apparent changes in flux are not associated with plume motion. Our method shows promise for understanding the time-history of plume dynamics at other hotspot ridges. Such constraints should improve our understanding of the dynamics of mantle plumes as well as the heat flow and geochemical structure of the mantle.iii