Pounds per square foot
Pounds per square foot (psf) is a unit used in the United States, United Kingdom and some other countries that use the Imperial system. It is the pressure of a one pound on an area of one square foot.
pound force per square inch
Pound force per square inch (psi) is the pressure from a force of one pound-force on an area of one square inch, which is about 6,894.76 pascals. Car tires in the United States are typically inflated to 30–35 psi.