The bite strength of a Bullmastiff has not been measured directly, but other dogs of similar size, such as Rottweilers, can bite with 328 pounds of pressure. For comparison, a lion's bite strength is 600 pounds. The bite strength ultimately depends on the size of the dog.
Studies of various dog breeds find that the shape of the head also plays a role in the power of the bite force. Small dogs with brachycephalic, or flattened, faces have much lower biting forces than small dogs with elongated faces. For larger dogs, having a brachycephalic face slightly increases the biting force.