Because psi is a measurement of pressure, and ft-lbs is a measurement of total energy, psi cannot be converted to ft-lbs. Neither is a major unit of measurement in the International System of Units. The units of measure psi and ft-lbs are mostly used in the United States and the United Kingdom.
In the International System of Units, psi and ft-lbs can be converted to their respective units. One psi is equal to 6,895 Pascals per metre, or 6,895 Newtons per metre. 1.356 Joules of energy are equal to one ft-lb. Engineer James Watt used the term to describe the amount of labour a horse can do in a given length of time.