Plantar heel pain is a common condition that is often exacerbated by the repetitive stresses of walking. Treatment usually includes an in-shoe intervention designed to reduce plantar pressure under the heel by using insoles and a variety of off-the-shelf products. The design process for these products is often intuitive in nature and does not always rely on scientifically derived guidelines. Finite element analysis provides an efficient computational framework to investigate the performance of a large number of designs for optimal plantar pressure reduction. In this study, we used two-dimensional plane strain finite element modeling to investigate 27 insole designs. Combinations of three insole conformity levels (flat, half conforming, full conforming), three insole thickness values (6.3, 9.5 and 12.7 mm) and three insole materials (Poron Cushioning, Microcel Puff Lite and Microcel Puff) were simulated during the early support phase of gait. Plantar pressures predicted by the model were validated by experimental trials conducted in the same subject whose heel was modeled by loading the bare foot on a rigid surface and on foam mats. Conformity of the insole was the most important design variable, whereas peak pressures were relatively insensitive to insole material selection. The model predicted a 24% relief in pressure compared to barefoot conditions when using flat insoles; the reduction increased up to 44% for full conforming insoles.