Write out the word “percent” when you refer to a percent or percentage in formal writing. Do not use the percent symbol in formal writing.
Use numerals to express numbers 10 and above. For example, write, “The probability of an earthquake in this region is approximately 15 percent” or “Roughly 50 percent of fossils found were from the cretaceous period.”
Express numbers under 10 with words. For example, write, “The success rate is only seven percent” or “Only one percent of the population falls into this category.”
Start sentences with words—never numbers. If a sentence begins with a number percentage, express the number in words or rewrite the sentence so the number does not start the sentence. For example, write, “Four percent of students failed the class” or “Sixty percent of the dogs ran away.”