Pounds per square inch cannot be converted to ft-lbs because psi is a measurement of pressure, whereas ft-lbs is a measurement of total energy. Neither one is in the International System of Units as a primary form of measurement. Psi and ft-lbs are primarily American and British units of measure.
Psi and ft-lbs can be converted into their respective units in the International System of Units. One psi equals 6,895 Pascals, which is the same as 6,895 Newtons per meter. One ft-lb is equivalent to 1.356 Joules of energy. It was invented by engineer James Watt to describe the amount of work a horse can perform over a specified period of time.