If you just googled “When to use percent or percentage,” you’ve come to the right place. Here’s what you need to know:
“Percent” and “percentage” are sometimes used interchangeably, especially when people are speaking informally. But these words actually have slightly different meanings (“percent” is one part of a hundred; “percentage” is a part of a whole) and are different parts of speech; “percent” is an adverb and “percentage” is a noun.
The Chicago Manual of Style, 17th Edition, our preferred style guide (mainly because anything over 1,100 pages must be right), has this to say on the issue:
“Despite changing usage, Chicago continues to regard percent as an adverb (“per, or out of, each hundred,” as in 10 percent of the class)—or, less commonly, an adjective (a 10 percent raise)—and to use percentage as the noun form (a significant percentage of her income). The symbol %, however, may stand for either word.” (3.82)