Answer:  To enter pounds per square inch, use the abbreviation psi or enter it as lbf/in^2, pound*g/square inch, or pound force/square inch.

Explanation:  ConvertIt recognizes pounds as a measurement of mass.  As such, it is compatible with kilograms and other measurements of mass.  When used in the phrase "pounds per square inch," pounds really means pounds of force, which is pounds of mass times the universal gravitational constant.  The universal gravitational constant can be entered using its abbreviation g.  g can be combined with other measurements such as ounces to produce measurements like "ounces of force."