percent, %, and percentage
Use the symbol % with numerals; use the word "percent" when you spell out numbers at the beginning of a sentence. To determine whether "percent" or % is singular or plural, look at the noun following it. If the next noun is a plural, use a plural verb; if it's singular, use a singular verb.
Six percent of the pipes were rusty.
More than 10% of that amount was allocated to planning.
When there is no number, use the word "percentage," unless people in your field use a different terminology, such as "percent difference."