Introduction: Foramen magnum is an important landmark at the junction of skull and spine. As it transmits structures like the lower end of the medulla oblongata, meninges, vertebral arteries and the spinal accessory nerve. Aims and objectives: Our aim was to measure anteroposterior and transverse diameters of foramen magnum and to observe its different shapes. Materials and methods: Thirty Four (34) adult human skulls of unknown sex were obtained from Department of Anatomy, MGM Medical College Jamshedpur. Diameters of Foramen magnum were calculated using sliding vernier calipers and shapes were assessed. Observations and results: Maximum anteroposterior diameter of foramen magnum was found to be 42.5mm and minimum was 27.5mm with the mean of 33.04mm whereas Maximum transverse diameter was 34mm and minimum was 23mm with the mean of 26.74mm. Percentage of foramen magnum shapes were as follows: oval 60%, Tetragonal 19%, hexagonal 12%, circular 9% and irregular 2% cases. Conclusion: Knowledge of dimensions of foramen magnum is important for radiologist, orthopaedicians and neurosurgeons, while planning and performing cranio-vertebral junction surgeries.