The normal maximum mouth opening (MMO) is approximately 48 millimeters for humans. The ability to position three fingers of either the left or the right hand in the mouth between the upper and the lower front teeth indicates normal size of the mouth.
Studies show that the maximum mouth opening depends on body size and height. Thus, men can open their mouths wider than women, and taller persons wider than shorter ones. Insertion of four fingers of the right hand in the mouth yielded an average MMO of 58.1 millimeters, while the width of four fingers of the left hand gave 57.5 millimeter, according to a 2003 study in the Journal of Canadian Dental Association.