The basic rule
Fewer is used with countable nouns — things you can count one by one: "fewer cars," "fewer mistakes," "fewer people." Less is used with uncountable (mass) nouns — things measured in bulk, not counted individually: "less water," "less time," "less money."
The trick
Ask: can I count these one, two, three? If yes ("one car, two cars"), use fewer. If it's a mass or an abstract amount ("water," "sugar," "time"), use less.
The famous exception
Amounts of money, distance, weight, and time are often treated as single units, not individually counted items, so "less" is standard: "less than $10," "less than five minutes," "less than 5 miles" — even though these look like numbers.
Examples
Fewer: "fewer than 10 items" (supermarket express lane), "fewer complaints this month."
Less: "less sugar in my coffee," "spend less time on your phone."
Frequently asked questions
What's the quick rule for fewer vs. less?
If you can count it item by item, use "fewer." If it's a bulk or abstract amount, use "less."
Why does "10 items or less" sound wrong to grammar purists?
Because "items" are countable, so strict grammar calls for "10 items or fewer" — though "or less" is extremely common in everyday usage.
Is "less than five minutes" correct even though minutes are countable?
Yes — time, money, and distance are treated as single measured amounts, so "less" is standard here, as an accepted exception.
Which is more formal, fewer or less?
Neither is inherently more formal, but using "fewer" correctly with countable nouns is seen as more grammatically precise.