Male haircuts will conform to the following standards.

(a) The hair on top of the head must be neatly groomed. The length and bulk of the
hair may not be excessive or present a ragged, unkempt, or extreme appearance.
The hair must present a tapered appearance. A tapered appearance is one where
the outline of the soldier’s hair conforms to the shape of the head, curving inward to
the natural termination point at the base of the neck. When the hair is combed, it will
not fall over the ears or eyebrows, or touch the collar, except for the closely cut hair at
the back of the neck. The block-cut fullness in the back is permitted to a moderate
degree, as long as the tapered look is maintained. In all cases, the bulk or length of
hair may not interfere with the normal wear of headgear or protective masks or
equipment. Males are not authorized to wear braids, cornrows, or dreadlocks
(unkempt, twisted, matted, individual parts of hair) while in uniform or in civilian clothes
on duty. Hair that is clipped closely or shaved to the scalp is authorized.

(b) Males will keep sideburns neatly trimmed. Sideburns may not be flared; the base
of the sideburn will be a cleanshaven, horizontal line. Sideburns will not extend below
the lowest part of the exterior ear opening.

(c) Males will keep their face clean-shaven when in uniform or in civilian clothes on
duty. Mustaches are permitted; if worn, males will keep mustaches neatly trimmed,
tapered, and tidy. Mustaches will not present a chopped off or bushy appearance,
and no portion of the mustache will cover the upper lip line or extend sideways
beyond a vertical line drawn upward from the corners of the mouth (see figure 1–1).
Handlebar mustaches, goatees, and beards are not authorized. If appropriate medical
authority prescribes beard growth, the length required for medical treatment must be
specified. For example, “The length of the beard will not exceed 1⁄4 inch” (see TB
MED 287). Soldiers will keep the growth trimmed to the level specified by appropriate
medical authority, but they are not authorized to shape the growth into goatees, or
“Fu Manchu” or handlebar mustaches.

(d) Males are prohibited from wearing wigs or hairpieces while in uniform or in civilian
clothes on duty, except to cover natural baldness or physical disfiguration caused by
accident or medical procedure.
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US Army Hair Cut Standards, Styles, and requirements