If you want a simple rule, the difference between "less" and "fewer" is straightforward: The traditional advice is that "fewer" is for things you count, and "less" is for things you don’t count.

You can count M&Ms, glasses of water, and potatoes—so you eat fewer M&Ms, serve fewer glasses of water, and buy fewer potatoes for the salad.

You can’t count candy, water, or potato salad—so you eat less candy, observe that the lake has less water, and make less potato salad for the next potluck.

The 'Singular Versus Plural' Rule

As I said, that's the simple rule, and the one you'll hear most often, but another way to think about the difference that also takes care of some of the exceptions to the simple rule is to use "less" for singular nouns and "fewer" for plural nouns. For example, the Chicago Manual of Style recommends using the “singular or plural” framework.

For the easy nouns, it works the same way:

Singular: Less Plural: Fewer Candy is . . . less candy M&Ms are . . . fewer M&Ms Water is . . . less water Glasses of water are . . . fewer glasses of water Potato salad is . . . less potato salad Potatoes are . . . fewer potatoes

Time, Money, Distance, and Weight

Time, money, distance, and weight are often listed as exceptions to the traditional “can you count it” rule because they take "less," but when you use the “singular or plural” rule, time, money, distance, and weight all fall in line. Although a thousand dollars is certainly countable—a bank teller will do it for you gladly—we routinely ignore that fact and think of them as singular amounts:

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He believes $1,000 is a lot of money.

a lot of money. She says that 50 miles is a long drive for ice cream.

a long drive for ice cream. We think 12 hours is too much time to spend on the road.

They’re singular and they take "less":

We had less than $1,000 in the bank.

than $1,000 in the bank. We’re less than 50 miles away.

than 50 miles away. I can fix the roof in less than 12 hours.

One Less Complaint

Using the “singular or plural” rule also explains another “exception.” People often think phrases such as "one less banana" are wrong because you can count bananas, but "one less banana" is correct because it is singular and you use "less" with singular nouns.

"One less banana" and similar phrases put you in a tricky situation because they are correct, but many people think they are wrong. For example, I got grammar-related complaints after Gardasil launched its “one less person affected with HPV” ads because many people thought it was grammatically incorrect. Therefore, I recommend avoiding the construction whenever possible. It’s better to rewrite your sentence than to have people think you’ve made a mistake or to knowingly use the wrong word by writing "one fewer X." You really can’t win whether you write "one less banana" or "one fewer banana." So rewrite. Instead of telling your caterer "We need one less banana in the fruit bowl," avoid the controversial sentence by asking her to "Take one banana out of the fruit bowl."

10 Items or Less

Finally, the simple and ubiquitous grocery store signs that read "10 Items or Less" aren’t the clear-cut abomination that many people believe them to be.

Although Garner’s Modern American Usage says that "10 items or fewer" is the correct choice, other reference books such as Merriam-Webster’s Dictionary of English Usage and The Cambridge Guide to English Usage note that the admonition that writers should not use "less" for countable items is relatively new, beginning as the personal opinion of one usage writer from the 1700s, and the Oxford English Dictionary has examples of "less" being used with countable items going back to nearly the dawn of printed English and continuing to this day. I find it impressive that the first citation of "less" being used with a countable noun in the OED comes from King Alfred the Great himself. He was the great promoter of English over Latin, and in the year 888, he wrote about "less words."

See Also: Why Are British and American English Different?

Language researchers tend to believe that using "less" with some countable nouns is natural and that the restriction against doing so is constructed and forced. For example, Mark Liberman reported on the linguistics site Language Log that in real writing—both from Google News and the Web in general—instances of “N votes or less” far exceeded “N votes or fewer.”

'Much' Versus 'Many'

Second, as with "less" and "fewer," "much" is generally used for things you can’t count and many is used for things you can count, but it is equally acceptable at the grocery store to ask both "How much can I bring through this line? Is this too much?" and "How many can I bring through this line?"

To me, the “how much” questions sound more natural, which would imply that we think of our items on the conveyor belt as a single uncountable mass of groceries rather than countable items—but you can make an argument for either.

What I ask is not that you use "10 items or less" in your own writing; it carries even more risk than using the "one-less-banana" construction. What I ask is that the next time you see a sign that reads "10 Items or Less," instead of getting upset about the sign, recognize that this isn’t a black-and-white issue and save your anger for something about which we can all agree: the people who go through that line with 40 items should be stopped.

See Also: "Much Thanks" or "Many Thanks"?