So the general rule is that if you’re talking about an amount of something that can’t be counted, it’s less and if you’re talking about a number of people or things, it’s fewer. Another way to remember the main distinction is to think along the following lines:

not as much = less

not as many = fewer

There’s always a ‘but’. . .

However, there are a couple of issues to be wary of. Firstly, having absorbed the guidelines above, you may suppose that some supermarkets are grammatically on the ball by displaying notices at checkouts that state ’10 items or fewer’ (fewer rather than less being the right choice because it’s referring to items, that is, a number of things?). In fact, there were reports a few years back that Tesco had replaced their signs reading ’10 items or less’ with ones which said ‘Up to 10 items’, so as to placate the sticklers. Sorry, no need! This is an example of hypercorrection. Pocket Fowler’s Modern English Usage puts it very succinctly:

‘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).’

Secondly, in sentences and phrases with ‘than’, you should use less with numbers when they are on their own:

√ His weight fell from 18 stone to less than 12. √ A person with a score of less than 100 will have difficulty obtaining credit.

and when talking about distance, time, ages, and sums of money:

√ Companies less than five years old are the ones bringing us new job creation. √ Per capita income is reckoned to be less than 50 dollars per year. √ Heath Square is less than four miles away from Dublin city centre.

But hold on, I hear you say – the measurements (years, miles, dollars, etc.) are in the plural, so why isn’t fewer the correct choice? Not so! We use less in such cases because we’re actually still referring to total amounts (of time, money, distance, etc.) rather than individual units.