less vs. fewer vs. under

When comparing quantities:

• Use less to refer to a mass amount, value, or degree.
• Use fewer to refer to a countable number of items.
• Don't use under to refer to a quantity or number.

Examples
The new building has less floor space and contains fewer offices.
The new building has fewer square feet of floor space and contains fewer offices.
Fewer than 75 members were present.
Less than a quorum attended.