With an "Inch" measuring tape or laser, measure the vertical dimension from finished (AFF) or estimated finished floor to horizontal stud (2x4). It's a good idea to measure in 3 places, left, center and right and choose the smallest dimension. Max rough opening height is 110" for a 108" tall door. For higher sizes please contact us directly at 888 308 9778.If you do not have a rough opening height dimension yet, you can always enter the dimension of the door slab on right cell and system will calculate automatically your required R/O dimension on the left