At full capacity is a commonly used phrase to describe a situation where something is operating at its maximum possible level. However, there are several synonyms for this phrase that can be used to convey the same meaning. One such synonym is "maximum utilization" which implies that something is being used to the fullest extent possible. Another synonym is "peak performance" which suggests that something is performing at its highest level. "Full utilization" is another synonym that signifies that something is being used to its fullest potential. Lastly, "maximum output" refers to producing the highest quantity of something possible. These synonyms can be interchanged with "at full capacity" to add variety and depth to written or spoken communication.