Captain (United States O-6)
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In the United States Navy, captain (abbreviated CAPT) is a senior officer rank, with the pay grade of O-6. It ranks above commander and below rear admiral (lower half). It is equivalent to the rank of colonel in the other uniformed services. Promotion to captain is governed by Department of Defense policies derived from the Defense Officer Personnel Management Act of 1980. DOPMA guidelines suggest 50% of commanders should be promoted to captain after serving a minimum of three years at their present rank and after attaining 21–23 years of cumulative commissioned service
Navy captains with sea commands in the surface warfare community generally command ships of cruiser size or larger. The more senior the officer, the larger the ship. In the submarine community, a captain will typically command a ballistic-missile submarine (SSBN), or a squadron of attack submarines (SSN). In naval aviation, captains with sea commands generally command aircraft carriers, air-capable amphibious assault ships, carrier air wings, functional air wings or special mission air wings or air groups. Commanders of aircraft carrier strike groups and expeditionary strike groups are normally rear admirals, while subordinate destroyer squadron commodores, amphibious squadron commodores, carrier air wing commanders and the individual ship commanding officers within the strike group are of captain rank or lower. In rare instances, the carrier air wing commander may be a Marine Corps colonel who is a naval aviator or naval flight officer; while in the expeditionary strike group, the Marine Expeditionary Unit commanding officer will be a Marine Corps colonel. Adding to the confusion, all Navy ship commanders are called "captain" regardless of rank.
Navy captains who are line officers may also fill important senior staff positions or have shore based command assignments, such as commanding officer of naval stations, naval air stations, naval air facilities, naval support activities, logistics groups, specialized centers or schools, or commanders of test wings or training air wings. Staff corps captains will command facilities and organizations appropriate to their designators, such as naval hospitals commanded by medical corps, dental corps, medical service corps or nurse corps officers; supply centers by supply corps officers; or trial service offices commanded by judge advocate general corps officers.
U.S. Coast Guard 
The United States Coast Guard also uses the naval rank system. A Coast Guard captain ranks above a commander and below rear admiral (lower half). The sleeve and shoulder board insignia are similar to the Navy insignia, with a lighter shade of blue with a gold USCG shield above the stripes. Coast Guard captains follow career paths very similar to their Navy counterparts, with seagoing officers typically commanding large maritime security cutters or high endurance cutters and aviators commanding coast guard air stations. Coast Guard captains will also command all types of major Coast Guard shore installations and activities, as well as serve as chiefs of staff for Coast Guard flag officers. The Coast Guard has no staff corps officers.
U.S. Public Health Service and NOAA 
In the United States Public Health Service, and in the National Oceanic and Atmospheric Administration Commissioned Corps, captains are senior non-combatant officers that serve as directors or ranking supervisors in their respective uniformed service corps. Seagoing NOAA captains will also command certain NOAA ships, while NOAA aviators will command NOAA flight operations activities. PHS rapid deployment force teams, containing 105 PHS physicians, nurses, and other medical professionals, are exclusively commanded by captains.
U.S. Maritime Service 
Although it exists largely as a maritime training organization, the United States Maritime Service also uses the rank of captain. Even though the Maritime Service is an auxiliary service, the grade is appointed by the President via the Secretary of Transportation, making it a federally recognized rank with corresponding paygrade of O-6.
|United States commissioned officer and officer candidate ranks|
|Pay grade / Branch of service||Officer
|Approximate insignia||(no universal insignia)|
|Air Force||Cadet / OT||2d Lt||1st Lt||Capt||Maj||Lt Col||Col||Brig Gen||Maj Gen||Lt Gen||Gen||GAF|||
|Army||CDT / OC||2LT||1LT||CPT||MAJ||LTC||COL||BG||MG||LTG||GEN||GA||GAS|
|Marine Corps||Midn / Cand||2ndLt||1stLt||Capt||Maj||LtCol||Col||BGen||MajGen||LtGen||Gen|||||
|Navy||MIDN / OC||ENS||LTJG||LT||LCDR||CDR||CAPT||RDML||RADM||VADM||ADM||FADM||AN|
|Coast Guard||CDT / OC||ENS||LTJG||LT||LCDR||CDR||CAPT||RDML||RADM||VADM||ADM|||||
|Public Health Service||||ENS||LTJG||LT||LCDR||CDR||CAPT||RADM||RADM||VADM||ADM|||||
|National Oceanic and
 Grade is authorized by the U.S. Code for use but has not been created
 Grade has never been created or authorized
|United States warrant officer ranks|
|Public Health Service|||||||||||
|National Oceanic and
 Grade is authorized for use by U.S. Code but has not been created
 Grade never created or authorized