Design hydraulic loading rates (HLRD) are used in specifying the area of the bottom of drainfield trenches required for onsite wastewater systems (OWSs). Our objective was to develop a method for estimating the HLRD based on soil and biomat hydraulic properties. We used a two-dimensional computer model to determine the steady flux through the trench bottom for the 12 USDA soil textural classes with 5 cm of wastewater ponded in the trench as an estimate of the performance under normal operating conditions. We used two sets of boundary conditions at the bottom of the soil profile: a deep water table and a shallow water table. We also tested how well the simple Bouma equation estimated the bottom flux. To estimate the HLRD, we took 50% of the steady trench bottom flux as a safety factor. Despite the wide range in saturated hydraulic conductivities of the soil textural classes (8.18–642.98 cm d−1), the steady flow through the bottom of the trench in these soils fell in a narrow range of 2.92 to 10.43 cm d−1. With a modification to account for unsaturated flow within the biomat, the Bouma equation produced remarkably accurate estimates of trench bottom flux for all soil textural classes. Based on our estimates of HLRD, we divided the soil textural classes into four groups. Our results show that medium-textured soils should have higher HLRD than has been assumed in some systems for estimating HLRD due to the importance of unsaturated flow in OWS hydraulic performance.