I have already written about when to use fewer and less but I would like to address this phrase specifically:
Many people don’t like it – they insist that it should be fewer than ten items. Arguments rage all over the internet and people mutter angrily at signs above supermarket checkouts.
This is my take on the debate:
The use of less in this phrase is fine. Everyone should calm down.
Less is correct in this phrase because we are thinking of a total amount rather than individual units. It’s the same reason we would say less than five days or less than £10,000. We wouldn’t say fewer than 18 years old or fewer than 50 miles. This reasoning applies when the phrase takes a slightly different form, such as ten items or less.
This is how Pocket Fowler’s explains it:
Supermarket checkouts are correct when the signs they display read 5 items or less (which refers to a total amount), and are misguidedly pedantic when they read 5 items or fewer (which emphasizes individuality, surely not the intention).
And to be technical about it, the full-size Fowler’s adds:
In phrases like the above, less is a pronoun, not an adjective.
If you have trouble determining when to use less or fewer, the best thing to do is remember that fewer refers to number and less refers to quantity.
- Oxford Dictionaries Blog
- Fowler’s Dictionary of Modern English Usage, 2015
- Pocket Fowler’s Dictionary of Modern English Usage, Online