Should the number 8 in the sentence below be written as a word rather than a numeral, since it is a number smaller than 10?
Your writing should be consistent and free of any jarring elements. If numerals are used for one of the percentages, they should be used for all of them. In this case, writing out the first number would likely trip up the reader.
The Canadian Style states, "Numbers modifying the same or similar items should be treated alike within a given passage. If numerals are to be used for any, they should be used for all."
Keep in mind that the type of document is also a factor. If you have a lot of numbers in the text—such as in a scientific or financial text—you should choose numerals and use the percent sign (e.g. 15%). In general texts, percent is usually written out (e.g. 15 percent) unless it is used adjectivally (e.g. a 15% bond).