Filling up is a commonly used phrase that refers to the process of making something full. There are several synonyms that can be used in place of filling up, such as replenishing, refilling, topping up, loading, brimming and saturating. These synonyms emphasize the act of adding something to a container or space until it reaches its capacity. Other synonyms include cramming, stuffing, and jamming, which are more informal and imply that something is being filled to maximum capacity or beyond. Whichever word is chosen, the meaning remains the same - to make something full or complete.