The foramen magnum is a large opening in the occipital bone of the cranium. Diameters of foramen magnum are important because vital structures passing through it and for sex determination of skulls. The dimensions of the foramen magnum are clinically important because vital structures passing through it may endure compression such as in cases of foramen magnum herniation, foramen magnum meningiomas and foramen magnum achondroplasia. The knowledge of foramen magnum diameters is needed to determine some malformations such as Arnold Chiari syndrome, which shows expansion of transverse diameter.