When do I use "less"?

A little morei

If you cannot easily put a number on the noun, it is a mass — and mass nouns take less. You would not say "three waters" or "eight patiences," so it is less water, less patience. Counted nouns (apples, people) take fewer instead.

The full entryii

More quick answersiii

• → When do I use "fewer"? Use fewer for things you can count: fewer chairs, fewer errors, fewer hours logged.
• → Is it "less people" or "fewer people"? "Fewer people." People are countable, so the correct word is fewer.
• → Is it "10 items or less" or "10 items or fewer"? Grammatically, "10 items or fewer." Items are countable, so they take fewer.
• → Is it "less calories" or "fewer calories"? "Fewer calories." A calorie is a unit you can count, so the right word is fewer.
• → Is it "under 10 people" or "fewer than 10 people"? "Fewer than 10 people" is the precise form — people are countable, so fewer fits.