Here are examples of where the two words are used incorrectly:
The town has less cars entering the centre, now that it has decided on a policy of pedestrianisation. [Incorrect]
The town has fewer cars entering the centre, now that it has decided on a policy of pedestrianisation. [Correct]
There are less runners in the race today. [Incorrect]
There are fewer runners in the race today. [Correct]
The express lane is only for shoppers with ten items or less. [Incorrect]
The express lane is only for shoppers with ten items or fewer. [Correct]
Less should only be used with mass nouns that cannot be counted, such as less money, less time, or less energy.
Less means not as much.
Fewer is used with countable things such as people, books, or clubs.
Fewer means not as many.
Over ten years ago, Tesco had signs in its stores saying “10 items or less” for customers wanting to avoid standing in lengthy queues.
This came to the attention of the Plain English Campaign. Their members came up with an alternative which was “Up to 10 items” to avoid the grammatical error.
Are there any exceptions to the rules above?
Well, yes there are!
The tendency is to use less than rather than less. This phrase is correct when less than is used before a plural noun.
He has less than 5 minutes to finish his speech.
They had less than three miles to complete the race.
She earns a salary of less than £20,000 per year.