The MadSci Network states that it takes roughly 1,700 PSI, or more than one million kilograms per square meter, to break a femur. They further explain that whether a femur breaks depends on how the applied force is spread out since it is calculated per unit area.
As noted by the American Academy of Orthopaedic Surgeons, the femur is the largest, longest and strongest bone in the human body. Moreover, that is why it takes a great amount of force to break it. Summit Medical Group notes that incidents resulting in a broken femur are rare; however, an injury may result from falling from a high height or high-speed collision.