Kilopound per square inch (ksi)

The kilopound per square inch is a unit of pressure equal to one thousand pounds per square inch, or 6.895 megapascals. Its symbol is ksi. It is not a pressure unit in ordinary use but a stress unit, and it is how American engineering states the strength of structural materials.

Structural steel is specified in it. The grade long known as A36 yields at 36 kilopounds per square inch, and the wide-flange steel that replaced it as the American standard yields at 50. Reinforcing bar of grade 60 yields at 60, and high-strength bolts are rated at 120 in tension. Those numbers are the vocabulary of American structural design in the way that 355 and 500 are in Europe.

Concrete uses the same scale downward. A common structural mix reaches four kilopounds per square inch in compression at twenty-eight days, high-performance mixes reach eight or more, and the specified figure appears on drawings, in the mix design and on the test cylinder report. Aluminium alloys sit between the two, a common heat-treated grade yielding at about 35.

Fracture mechanics extends the unit in an unusual direction. Fracture toughness is quoted in kilopounds per square inch times the square root of an inch, a compound that looks strange but is dimensionally correct, and it describes how large a crack a material will tolerate before it runs. The metric equivalent is megapascals times the square root of a metre, and converting between them requires care.

Its relationship to the metric world is simple in principle. One kilopound per square inch is 6.895 megapascals, so a fifty ksi steel yields at 345 megapascals and a sixty ksi bar at 414. Anyone reading drawings from both traditions ends up learning that factor, because the two systems name the same steels with different numbers.

The name is sometimes misread. A kilopound here is a thousand pounds of force, not the kilopound of mass sometimes called a kip in shipping, and the pressure it names has nothing to do with any weight in a container. Reading ksi as a stress rather than as a load avoids the confusion.

One kilopound per square inch equals 1000 pounds per square inch, about 6.895 megapascals, about 68.95 bar, or about 68 atmospheres.

Kilopound per square inch conversion table