Kilopascal (kPa)

The kilopascal is a unit of pressure equal to one thousand pascals. Its symbol is kPa. It is a hundredth of a bar and ten hectopascals, and it is the coherent SI unit that engineering was meant to adopt when the pascal proved too small to write comfortably. Standard atmospheric pressure is 101.325 kilopascals.

Tyres are where most people meet it. Canada, Australia, New Zealand and much of Asia state recommended tyre pressures in kilopascals, so a car door placard reads 220 or 240 rather than 32 or 35 pounds per square inch. The two scales sit side by side on most modern gauges, and the conversion is close to seven kilopascals to the pound per square inch.

Civil engineering uses it for loads on ground and structure. The bearing capacity of a soil is quoted in kilopascals, a firm clay allowing perhaps 150 and a soft one much less, and wind loading on a facade is calculated in the same unit. Because a kilopascal is also a kilonewton per square metre, structural calculations move between force and pressure without a conversion factor.

Medicine uses it in one important place. Arterial blood gases are reported in kilopascals in Britain, Ireland and much of Europe, where a healthy oxygen partial pressure is eleven to thirteen, while the United States reports the same measurement in millimetres of mercury as eighty to a hundred. The two numbers describe identical blood, and a clinician reading a foreign chart must know which convention it follows.

Vacuum work counts downward in it. A rough vacuum is a few kilopascals absolute, a domestic vacuum cleaner pulls perhaps twenty kilopascals below atmospheric, and freeze-drying operates well under one. Stating vacuum as an absolute pressure in kilopascals avoids the ambiguity of describing it as a negative gauge pressure.

Its awkwardness is only rhetorical. A hundred kilopascals is exactly one bar and reads less neatly, which is why the bar has held on in Europe despite decades of official preference for the pascal. Where the kilopascal has won, as on Canadian and Australian tyre placards, it has done so because the number is printed rather than spoken.

One kilopascal equals 1000 pascals, 0.01 bar, 10 hectopascals, or about 0.145 pounds per square inch.

Kilopascal conversion table