Anyone know about how hard you have to hit your head before going into a coma?

Depends: There is no hard and fast rule. It depends on how much trauma or force is applied to the brain. There is no specific number or grading scale to measure how hard you have to be hit to cause enough damage to the brain to go into a coma.

Unpredictable: There are too many variables involved in brain injury and trauma to determine a specific amount of force necessary to predictably cause a severe brain injury that would result in coma, without it causing death. What ever prompted the question, don't do it.

Varies: Some apparently minor traumatic injuries can lead to severe brain injury from bleeding or swelling of the brain from it hitting on the inside of the skull. Age plays a part in this as does a history of prior injury. Some rather hard hits can be tolerated for reasons we don't understand.