Field studies of arthropod abundance may also need to know biomass. Biomass is used in studies of food web structure and tropic cascades, and for estimating vertebrate food sources and plant damage from herbivory. Weighing individual insects is time-consuming and body length is a more practical field measurement. We develop an allometric equation for arthropod biomass (wet and dry weight) as a function of length for application to field studies in the Pacific Coast region, USA. Our results are consistent with prior all-taxa equations.