To measure population density, the appropriate unit is typically people per square unit. In the context of your response, you would use "per square foot" if measuring a very small area, but more commonly, population density is expressed in people per square kilometer or people per square mile.
So, while "sq. foot" could be part of the context when discussing a measurement area, the complete measurement would ideally be "people per square foot" or another relevant combination that includes a population count along with the area measurement.