Mauna Loa, the world's largest volcano and greatest single land mass, has had previous estimates of its volume at 60,000 to 80,000 km3. We present a new estimate for the total eruptive volume of Mauna Loa calculated from Digital Elevation Models (DEMs) using GIS. We incorporate both the subaerial (10 m DEM) and subaqueous (100 m DEM) surfaces of the volcano. In order to attain a higher level of accuracy than previous estimates, our volume calculation also includes the volcano's "root" to account for the amount Mauna Loa depresses the underlying oceanic crust. Mauna Loa was isolated from the other volcanoes of the island of Hawai`i using ArcInfo GIS coverages describing the previously mapped geology and surficial extent of the island. These coverages were used to clip the DEMs in order to provide a mathematically queriable surface for the subaerial extent of Mauna Loa. The subaqueous portion of Mauna Loa extends out onto the Pacific plate to the point at which the seafloor becomes relatively flat. Seismic refraction data from previous studies were used to constrain the base of Mauna Loa's volcanic pile. Total volume was calculated by inverting the merged subaerial and subaqueous DEMs and adding an approximate volume contributed by the portion of the volcano that lies beneath the seafloor and deforms the oceanic lithosphere.