Fill up is a commonly used phrase that refers to making something full or complete. However, there are several different synonyms that can be used when talking about filling up. Some interchangeable phrases that can be used instead of fill up include top up, load up, pack in, cram in, stuff, brim, and replenish. These different words can be used to add clarity, variety or emphasis to your language and indicate how much you are filling up or what you are filling up with. So the next time you need to describe how you're filling something up, try using one of these synonyms and see how it changes your sentence.