Fewer versus less and amount versus number

What are the usage rules when using “fewer” and “less”, “amount” and “number” in a sentence? Believe it or not, the rules are simple.

A good place to start is to define each word and then think about the noun that you’re referring to and whether it’s singular, plural, or countable.

When referring to how much of something there is, use “amount” and “less”.

For example, when talking about water:

• How much water do we need?
• We need to determine the amount of water needed.
• We need less water than yesterday.

On the other hand, when talking about how many of something, use “number” and “fewer”.

For example, when talking about people:

• How many people are coming?
• We need to decide the number of people coming.
• There are fewer people coming than we thought.

Therefore, if you are stuck between the two, take whatever the subject is (water and people in the above examples) and ask “how much x do I need?” and “how many x do I need?”. You can also look at whether the item is countable. For example, you can count people, but you cannot count water. Therefore, if you can count it, use “number” and “fewer”, and if you cannot count it, use “less” and “amount”.