
If you are writing a technical or scientific paper, you would use the % sign. But in general or nonscientific writing, spell out the word percent. Except at the beginning of a sentence, percentages are usually expressed in numerals. You never want to begin a sentence with a numeral, so either rewrite so that doesn’t occur or spell out the number. Here are some good examples of correct usage:
- Fewer than 5 percent of readers buy books at an actual bookstore.
- With 90–95 percent of the work complete, we can relax.
- A 75 percent likelihood of winning is worth the effort.
- Her five-year certificate of deposit carries an interest rate of 5.9 percent.
- Only 20% of the ants were observed to react to the stimulus.
- The treatment resulted in a 20%–25% increase in reports of night blindness.
Percent, used as an adverb, is not interchangeable with the noun percentage (I know that 1 percent is a very small percentage). Note also that no space appears between the numeral and the symbol %.