There seems to be confusion about when to use the word less and when to use the word fewer. Fewer should be used when you are talking about things that can be counted. Grammar Girl calls them “count nouns.”
- He took three pencils and left fewer than four on her desk.
Less is used when you are talking about things that cannot necessarily be individually counted. Grammar Girl calls those “mass nouns.”
- If he used less sarcasm, he might have more friends.
Of course, we are talking about the English language, so there are exceptions. The word less is typically used for measurements of time, money, and distance.
- He had less than four hours of work left before his vacation.