Conversion from Kilopounds per square inch to Hectopascals

Information about the 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.

Information about the Hectopascal (hPa)

The hectopascal is a unit of pressure equal to one hundred pascals. Its symbol is hPa. It is exactly one millibar, and that identity is the whole reason it exists: when meteorology moved to SI units in the 1980s, the hectopascal let every barometer, chart and forecast keep its numbers unchanged while changing the name on them.

Weather is its home and almost its only use. Standard atmospheric pressure at sea level is 1013.25 hectopascals, a settled high-pressure system reads 1020 to 1035, and a deep Atlantic depression can fall below 950. Isobars on a synoptic chart are drawn at four-hectopascal intervals, so the spacing of the lines is a direct picture of how hard the wind will blow.

Storm intensity is reported the same way. The central pressure of a tropical cyclone is the single most quoted measure of its strength, and the lowest sea-level pressure ever recorded was 870 hectopascals in Typhoon Tip in 1979. A drop of a few tens of hectopascals over a day is enough to turn ordinary weather into a severe event.

Altitude changes the reading steadily. Near sea level pressure falls by roughly one hectopascal for every eight metres of height gained, which is why a barometric altimeter works at all and why a barometer at home responds to being carried upstairs. That gradient weakens with altitude, so the relationship holds only in the lowest few kilometres.

Aviation depends on the unit for safety. Pilots set their altimeters to a local pressure figure given in hectopascals so that everyone in the same airspace measures height from the same datum, and above a defined transition altitude all aircraft switch to the standard setting of 1013 hectopascals so that vertical separation is preserved regardless of the weather. North American practice uses inches of mercury for the same purpose, and the two must never be confused.

For conversion, one hectopascal is one millibar exactly, 0.1 kilopascals, and about 0.0295 inches of mercury. The last of those is the figure to watch, since a reading of 30 inches of mercury and one of 1016 hectopascals describe the same afternoon.

One hectopascal equals 100 pascals, 1 millibar, 0.1 kilopascals, or about 0.0295 inches of mercury.