English uses two lexemes to denote that something is smaller in number or size/amount: "Less" and "fewer". "Less" is used for uncountable nouns ("I needed less time to mow the lawn today"), while "fewer" is used for countable ones ("I needed fewer attempts to get the lawnmower going today").

However, sometimes "less" is used with countable nouns (famously the "X items or less" register in supermarkets). When can countable nouns be used with "less"? Consider these examples:

  • I'll only need five minutes or less to mow the lawn.
  • A lawnmower may only go 5 mph or less.
  • You may only use my lawnmower if you weigh 75 kg or less.

These sentences sound correct to me, while the use of "fewer" in these contexts seems incorrect or at least odd to me (as a non-native speaker of English):

  • ? I'll only need five minutes or fewer to mow the lawn.
  • ? A lawnmower may only go 5 mph or fewer.
  • ? You may only use my lawnmower if you weigh 75 kg or fewer.

Which of these do native speakers prefer?

Could it be that "less" is acceptable, when the countable noun refers to an uncountable entity (such as time, speed or weight)? If so, is this restricted to units (minutes, mph, kg)? Also: Would this explain the use of "less" in "X items or less", as "items" as a countable noun refers to an abstract, uncountable entity (i.e. "shopping")?

  • I think this is purely a usage issue, so the distinction is arbitrary. – Jeremy Needle Aug 7 '14 at 14:29
  • One possible explanation of the checkout line sign is that 'less' is shorter than 'fewer' and fits on the sign better. 'Less' isn't perfect with discrete items, but it works better than 'fewer' does with continuous things. – Mitch Aug 8 '14 at 13:26

The general principle here is that less should be used when describing a continuous quantity and fewer should be used for a discrete quantity. This is not the same as countable v non-countable.

Time is a continuous quantity, so "5 minutes or less" is correct.

A purchase is a discrete quantity, so "10 items or fewer" is correct.

Similarly, in your examples, speed and weight are continuous quantities.

As to the case of supermarket checkouts, there have been several campaigns in Britain to boycott those supermarkets that have "10 items or less" signs. With some success. It is now common to see "Up to 10 items" or "Maximum 10 items" in order to avoid the argument altogether.

In practice however, less is becoming increasingly used for all comparisons. In a similar vein, superlatives are becoming increasingly used in place of comparitives. It would be unnecessarily pedantic nowadays to insist upon one form over another.

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In the fewer/less conundrum, use less when the object in question is singular (or collective); use fewer the object is plural.

Your preferred examples seem to contradict that rule, but they don't.

I can't for the life of me remember what they are called, but in the examples you offer, the apparent countable with a finite number attached to it is considered a singular unit. You would correctly use your preferred examples, as you would be with following examples:

Five minutes (or less) is all I require to mow the lawn.

Five miles per hour (or less) is this lawnmower's top speed.

Seventy-five kilograms (or less) is the weight capacity of my lawnmower.

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This is a very good question. I too would definitely say “less”, not “fewer” in your three examples, but it is difficult to say exactly why this is the case. I suspect (as you do) that it has to do with the fact that we are measuring something (time, speed, weight), rather than actually counting objects (minutes, miles, kilograms).

On the other hand, I regard “six items or less” as a barbarism. But there is clearly a tendency in modern English, on both sides of the Atlantic, to replace “fewer” by “less” in all contexts.

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Generally speaking fewer should be used with nouns in the plural:

Consumers are buying fewer desktop PCs due to the power of mobile devices.

Fewer than 50 people attended the concert.

Fewer people are purchasing printed periodicals.

Less is used with uncountable nouns or with nouns that do not have plurals.

Parents feel they have less time to spend with their children now-a-days.

Most public schools have less money in their budgets than they did just a decade ago.

California has been getting less rain this summer than it did last summer.

Less is commonly used with measurements (especially time) and with numerals standing on their own.

5 is less than 25.

Their marriage lasted less than 75 days.

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This final case is why many people say 15 items or less. In their mind they are considering it to be a measurement. Time is also a measurement so it follows this rule as well.

Oxford Dictionaries: Language Matters

Wikipedia: Fewer vs. Less

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