Both打算 and 计划 can be used as nouns and translated as “plan”.
Both打算 and 计划 can be used as verbs and translated as “plan to”, usually followed by time adverbials.
计划：When used as a noun, it means “schedule”. It can be the object of制定/做 (make+计划) to express the specific and settled schedule, and it sounds more formal. In this way, it cannot be used interchangeably with 打算.