Is it 75 square feet or 75 square foot?

Question

Is it 75 square feet or 75 square foot?

Answer 1

It depends on what you are saying. For instance:

What are the measurements of a rectange if the area equals 75 square feet?

There was a 75-square-foot hole in the ground.

Answer 2

I would say "The size of the flower bed is 75 square feet."

However, "The 75-square-foot flower bed is quite pretty."

Hope that makes sense.