1 ksi = 1 psi * 1000
1 psi = 1 lbf / 1 in2 (pound per square inch)
Ksi are used where the pressure measured is a large value of psi. Ksi finds its application mostly in materials science to measure the tensile strength of metal beams. For example, A36 steel has a tensile strength of 58 ksi. They are not that widely used to measure gas pressures.
Psi is an imperial unit of pressure measurement most commonly used in the United States. Its corresponding metric unit is the Pascal which is widely used in most countries throughout the world.
One kilopound per square inch (ksi) is approximately 6894757 Pa.