The recent invasion of the pheretimoid earthworms in North America, specifically Metaphire hilgendorfi, Amynthas agrestis, and Amynthas tokioensis, commonly referred to as “jumping worms”, has led to an increase in research on their impacts on forest ecosystems, trophic interactions, and soil health. Determining the biomass of earthworms is a fundamental method in studies evaluating the impact of invasion. However, earthworm biomass is highly influenced by moisture content of a particular habitat, thus requiring ash-free dry-mass methods for accurate measurement. This method is time-consuming and results in the destruction of specimens. While an equation exists for Metaphire hilgendorfi, here we present novel allometric equations for converting length to ash-free dry mass for the 2 most common species of the trio, Amynthas agrestis and Amynthas tokioensis. The relationship between length and biomass was significantly different between A. agrestis and A. tokioensis. This relationship was also different for clitellate and non-clitellate A. tokioensis. Sampling site was an important factor in the biomass of A. tokioensis, suggesting that landscape context may be an important driver in the annual growth and development of this species. These allometric equations will support much needed research on the species-specific impacts of these pheretimoid earthworms on native and ornamental plants, soil structure and chemistry, and forest ecosystems.