Determining the shape of plant cellulose microfibrils is critical for understanding plant cell wall molecular architecture and conversion of cellulose into biofuels. Only recently has it been determined that these cellulose microfibrils are composed of 18 cellulose chains rather than 36 polymers arranged in a diamond-shaped pattern. This study uses density functional theory calculations to model three possible habits for the 18-chain microfibril and compares the calculated energies, structures, 13C NMR chemical shifts and WAXS diffractograms of each to evaluate which shape is most probable. Each model is capable of reproducing experimentally-observed data to some extent, but based on relative theoretical energies and reasonable reproduction of all variables considered, a microfibril based on 5 layers in a 34443 arrangement is predicted to be the most probable. A habit based on a 234432 arrangement is slightly less favored, and a 6 × 3 arrangement is considered improbable.