Accurate estimation of soil macroinvertebrate fresh biomass is crucial to link macroinvertebrate community to ecosystem functions, but remains a challenging task under field conditions. Here, we present allometric equations to estimate the fresh biomass of three diplopods (Rhinocricidae), one earthworm (Glossoscolescidae) and one earwig species (Anisolabididae) that are abundant in soil communities and potentially important for the provision of soil ecological functions in tropical agroecosystems. Body length, body width, and body volume, were measured using a novel method of image analysis, and then used to estimate the fresh biomass. Our results show that length-biomass allometric relationships provide reliable estimation of fresh biomass for diplopods (r2 = 0.98) and earwigs (r2 = 0.97). However, the biomass of earthworms was not as accurately predicted by body length (r2 = 0.82). The use of body volume, estimated with body length and width, allowed to increase the predictive power for earthworms. Furthermore, a general allometric equation based on body volume, including all taxa considered in this study, was found to predict 96% of the observed body weight variability, suggesting that this equation could be generalizable to a large range of soil macroinvertebrates. Therefore, we conclude that using body volume could provide a better accuracy in estimating soil macroinvertebrate biomass. Although the estimation of body volume on each individual requires an additional measure, the use of image analysis software renders this step feasible for a large number of individuals. By improving the feasibility of trait measurements, this method may facilitate field surveys and foster trait-based studies on soil macroinvertebrates.