4'11
and yes 8'15 is....something, plus you got it incorrect :/ 2.54 centimetres is one inch and so 30.48 centimetres is one foot, and also there are 12 inches in one foot so 15 should be 3 and add one to the 8 so youre saying 9'3 and that person now holds the world record because the tallest known man is 8'1 :/
so a regular 12 year old boy would be on average about 5'1, and grown up men are on average 5'10-5'11