Use the word fewer if you can count something:
- She said her apartment was infested, but the exterminators counted fewer than seven ants in the entire complex.
- I got fewer cookies than you did.
- You are drinking fewer glasses of wine at dinner than you used to.
Use less if you cannot count something:
- You are drinking less wine than usual.
- It looks like there’s less snow on the mountain this year.
However, nothing is ever that simple. So …
Even though you can count up all the money, coins and bills, in my purse, the $5 total is considered a bulk amount in this sentence:
- I have less than $5 in my purse.
- I think I have fewer than five quarters in my purse for the parking meter. (I’m talking about each individual quarter.)
When indicating a span of time rather than the individual number of years or individual minutes, use less:
- I graduated college in less than four years.
- She completed the exam in less than 30 minutes.
Same with distance:
- The office is less than five miles away.
- The lamb weighs less than 100 pounds.