United States Code explicitly limits the total number of vice admirals that may be on active duty at any given time.

The President may also add vice admirals to the Navy if they are offset by removing an equivalent number of three-star officers from other services.

[2] Finally, all statutory limits may be waived at the President's discretion during time of war or national emergency.

Occasionally, the superintendent of the academy is appointed to the rank vice admiral due to their longevity in the position, or for prior military experience.

[12] Vice admirals are nominated for appointment by the President from any eligible officers holding the rank of rear admiral (lower half) or above, who also meet the requirements for the position, under the advice or suggestion of the Secretary of Defense, the applicable service secretary, or the Chairman of the Joint Chiefs of Staff.

Three-star officers must retire after 38 years of service unless appointed for promotion or reappointed to grade to serve longer.

Flag officers typically retire well in advance of the statutory age and service limits, so as not to impede the upward career mobility of their juniors.