The requirements  for military hair grooming standards are necessary
    to maintain good order and discipline. Many styles of military haircuts
    are acceptable by the Army, Navy, Air Force or USMC as long as they
    are clean, neat and conservative.  However, remember that all military
    services have slightly different requirements for haircuts.

    The most basic military haircut requirements require that sideburns do
    not exceed the ear opening, that the hair be neatly tapered and that
    bulk normally not exceed 2 inches. Further  military haircut standards
    for the Army, Navy, Air Force and USMC are below.
       
Military-Haircuts.com
Army, Navy, AF and CG Hair Cut Standards, Styles, and requirements