Copyright © 2019 Wolters Kluwer Health, Inc. Objective:To determine whether patient height correlates with implant length selection of antegrade or retrograde femoral intramedullary implants.Design:Case-control Study.Setting:A single level 1 trauma center.Patients/Participants:All patients with operatively treated femoral shaft fractures from 2011 to 2017.Intervention:All fractures were treated with an intramedullary nail. Retrograde or antegrade insertion was decided at the discretion of the surgeon and fracture location.Main Outcome Measurements:Length of femoral implant and patient height.Results:Five hundred sixty-seven operatively treated fractures were reviewed, 322 antegrade and 245 retrograde. The correlation between patient height and actual nail length was 0.55 for antegrade implants (P < 0.01) and 0.59 for retrograde implants (P < 0.01). Separate prediction equations for nail length based on patient height were estimated for antegrade and retrograde implants and found accurate prediction of length 86% and 88% of the time, respectively.Conclusions:Intramedullary nail length can be accurately predicted based on patient height and the technique using the equations below. This is the first study to establish a simple equation to serve as an adjunct for selecting the most appropriate length implant. This equation can be used in cases of bilateral femur fractures, templating to have appropriate sized implants in close proximity to the odds ratio, and in remote environments where surgical planning is critical for determining implant needs.