Background The CT details of the dimensions of the posterior ethmoid sinus are not fully covered in the literature, so building up for a base for the CT measurements and description of that area is important. Preoperative details of the posterior ethmoid sinuses are mandatory before any approach or procedure involving this area . Objective To determine the different dimensions, measurements, and grading of the posterior ethmoid sinus by computed tomography (CT) that were not previously published. Methods Two hundred paranasal CT scans (400 sides) were included in the study. Axial images were acquired with multiplanar reformats to obtain delicate details in coronal and sagittal planes for all subjects. Results Within 200 CTs (400 sides), the mean anteroposterior dimension of the posterior ethmoid was 13.62± 1.75 mm (range= 9.5–19.5), the mean posterior ethmoid transverse diameter was 12.15+1.6 mm (range= 8–16.2) and the mean posterior ethmoid height was 44.64±3.83 mm (range= 35.8–56) without reported significant differences between both sides in all posterior ethmoid dimensions. The mean width of the posterior ethmoid sinus and its height from the orbital roof and nasal floor was significantly more in males than in females. There was significantly lower fovea ethmoidalis in males than females as we go posterior. Conclusion This study improves surgeons’ awareness and orientation of posterior ethmoid sinus variations in the endoscopic sinus surgery and can be of help to residents in training.