Use fewer when you are talking about a smaller number of countable things.

Use lesser when you mean smaller in importance, rank, degree, or significance.

In other words:

So this is not really a fewer vs. less problem. It is a different contrast:

✅ We had fewer meetings this week.

Here, meetings can be counted, so fewer is the right choice.

✅ Fewer students signed up this year.

Again, students are countable, so fewer works.

✅ He was charged with a lesser offense.

Here, lesser means a smaller or less serious degree, not a smaller number.

✅ The committee ignored the lesser issues and focused on the main problem.

In this sentence, lesser means less important, not fewer in number.

✅ There were fewer complaints, but the remaining ones involved lesser problems.

This shows the difference clearly:

The most common mistake is using lesser as if it were just a fancy version of fewer.

It isn’t.

For example:

❌ We need lesser people in this room.
✅ We need fewer people in this room.

Why? Because people are countable, so the issue is number.

Another common mistake is thinking lesser is always interchangeable with smaller. Sometimes it works that way, but lesser often has a more abstract meaning tied to:

So lesser is often more about hierarchy or degree than about physical size or count.

Use this fast test:

Can I count it one by one?

A simple memory trick:

Or even shorter:

fewer counts
lesser downgrades