COMDTINST 1020.8I separates a member after three consecutive failed semiannual Body Composition Screenings (BCS), the "three-strike rule." A single bad failure can also skip probation entirely: exceeding the limits by wide margins, or a third non-compliant BCS within 14 months, sends a member straight to separation processing.
The Coast Guard's weight and body fat rules are set by COMDTINST 1020.8I, the Body Composition Standards Program, which applies to all active duty and reserve military personnel and to Public Health Service officers detailed to the Coast Guard.
Compliance is checked twice a year, and three consecutive misses is a named rule with its own name:
Members who fail to maintain compliance with body composition standards three consecutive semiannual BCSs (Apr-Oct-Apr or Oct-Apr-Oct), also known as the three-strike rule.
COMDTINST 1020.8I, para 4.A.5
A failed BCS taken while already on probation still counts as one of the three strikes, and a member who racks up a third strike while on probation is separated regardless of how much time is left in that probationary period.
Probation is not automatic. A member can be found ineligible for it outright, which shortens the path to separation:
Members who are non-compliant for a third consecutive time within a 14-month period are ineligible for probation and will be processed for separation.
COMDTINST 1020.8I, para 3.D.3.b
The same paragraph makes a member ineligible for probation, and headed straight for separation processing, if they miss the standards by wide margins on a single BCS: more than eight percent over the maximum allowable body fat, more than four inches over the abdominal circumference limit, and more than 35 pounds over the maximum screening weight, unless they pass the Physical Fitness Test.
Any one of those, on its own, is grounds for a command to recommend separation; a member does not need to hit all of them.