The USAF answer is as follows:
AFI36-2903 29 SEPTEMBER 2002
Table 2.5. Body Physical Appearance Modification Standards.
(2) Tattoos/Brands Unauthorized (content): Tattoos/brands anywhere on the body that are obscene, advocate sexual, racial, ethnic, or religious discrimination are prohibited in and out of
uniform. Tattoos/brands that are prejudicial to good order and discipline or that are of
a nature that tends to bring discredit upon the Air Force are prohibited in and out of
uniform. Unauthorized (content): Any member obtaining unauthorized tattoos will be
required to remove them at his/her own expense. Using uniform items to cover
unauthorized tattoos is not an option. Members failing to remove or alter unauthorized
tattoos in a timely manner may be subject to disciplinary action or involuntary
separation. Failure to observe these mandatory provisions and prohibitions by
active duty Air Force members, USAFR members on active duty or inactive duty
for training and ANG members in Federal service is a violation of Article 92,
Uniform Code of Military Justice (UCMJ). Inappropriate (military image):
Excessive tattoos/brands will not be exposed or visible (includes visible through the
uniform) while in uniform. Excessive is defined as any tattoo/brands that exceed ¼ of
the exposed body part and those above the collarbone and readily visible when wearing
an open collar uniform. Members should not be allowed to display excessive tattoos
that would detract from an appropriate professional image while in uniform.
Commanders should use these guidelines in determining appropriate military image
and acceptability of tattoos displayed by members in uniform. Air Force members with
existing tattoos not meeting an acceptable military image should be required to (a)
maintain complete coverage of the tattoos using current uniform items (e.g.
long-sleeved shirt/blouse, pants/slacks, dark hosiery, etc.) or (b) remove tattoo(s).
Depending on the circumstances, commanders may seek Air Force medical support for
voluntary tattoo removal. Members failing to remove or alter tattoos or who choose not
to comply with acceptable military standards may be subject to disciplinary action or
involuntary separation. Failure to observe these mandatory provisions and
prohibitions by active duty Air Force members, USAFR members on active duty
or inactive duty for training and ANG members in Federal service is a violation
of Article 92, Uniform Code of Military Justice (UCMJ). (See notes 2, 3, and 4).
Bottom line is: As long as it's not in bad taste, it can't cover more than 1/4 of the exposed body part.