The geometric effects of iso-Q sample – a geometry that is widely used in arc-jet experiments – were numerically studied on charring ablative materials under arc-jet conditions. The objective was to investigate how the sample diameter, thickness, and sample holder affect the pyrolysis gas transport, thermal, and material response within the material. A series of three-dimensional (3D) simulations was performed on different iso-Q samples. Identical conditions were applied on the same low-density charring ablative material, but with different sample diameters and thicknesses. The results indicate that the sample diameter influences both side-wall heating behavior and the amount of gas blowing through the front surface; the sample thickness significantly affects the side-wall blowing pattern. In addition, two types of impermeable sample holders were studied. Sample holders which sheath the outside of the sample were found to result in effects similar to shortening the depth of the sample. Moreover, for samples with high diameter-to-depth ratio, the sample holder was found to increase the blowing rate at the stagnation point. These results can be used as general guidelines for choosing model geometries in future arc-jet experiments. The importance of 3D modeling and the limit of the one-dimensional (1D) assumption made in most material response codes, are also addressed.