Convert pressure

About the units

• Pascals (Pa) – The pascal is the SI coherent unit of pressure and stress, one newton per square metre, named by the General Conference on Weights and Measures in 1971 and legal throughout the EU. It is a small pressure – paper of 80 grams per square metre presses on a desk with just under 0.8 pascals – so prefixes are nearly always used: hectopascals for weather, kilopascals and megapascals in engineering. A newton per square millimetre is exactly one megapascal. The symbol is Pa, with a capital P.
• Hectopascals (hPa) – A hectopascal is 100 pascals and exactly one millibar, so the two names give the same figure. It is the unit of weather charts, and the Met Office notes that near the ground pressure falls by about 1 hPa for every 10 metres of height. Standard sea-level pressure is 1,013.25 hPa, and Swedish records span 938.4 to 1,063.7 hPa. The trap is the kilopascal, ten times larger: engineering writes the same 1,013 hPa as 101.3 kPa.
• Kilopascals (kPa) – A kilopascal is 1,000 pascals. The pascal is the SI coherent unit of pressure, one kilogram per metre per second squared, and the name was adopted by the General Conference on Weights and Measures in 1971. The unit is legal across the EU and is used in engineering, geotechnics and medicine, and in some countries for tyre pressure. A hundred kilopascals is exactly one bar and 1,000 hectopascals. Normal air pressure is 101.325 kilopascals. Weather charts use hectopascals and engineering kilopascals.
• Bar (bar) – One bar is exactly 100,000 pascals. The unit was taken up by the General Conference on Weights and Measures in 1948 and appears in the EU units directive, so it is legal across the union. It is used for tyre pressure, plumbing, diving and process industry in Europe, and in weather reports as the hectopascal, which is the same thing as the millibar. One bar is 14.5 psi. Normal sea-level pressure is 1.01325 bar, so not quite one bar, and tyre pressure is gauge pressure.
• Atmospheres (atm) – One standard atmosphere is exactly 101,325 pascals, fixed for general use by the General Conference on Weights and Measures in 1954. That is 1.01325 bar or 14.7 psi, and the value comes from the mercury barometer and its 760 millimetres. The atmosphere serves as a reference pressure in chemistry, diving and materials testing, but it is absent from the EU units directive and is not a legal trade unit. The technical atmosphere is a different unit, 98,066.5 pascals, some three per cent lower.
• Psi (psi) – Psi is pounds-force per square inch and has no definition in the SI or in EU law. The value follows from the exact mass of the pound and standard gravity of 9.80665 metres per second squared, and comes to 6,894.757 pascals. It is used for tyre and process pressure in the US, where the federal fuel economy site works in psi as well, while in the EU it may only be a supplementary indication. One bar is 14.5 psi, not 14. Keep psig, gauge pressure, apart from psia, absolute pressure.
• Millimetres of mercury (mmHg) – A millimetre of mercury is the pressure of a mercury column one millimetre high, and the unit of blood pressure. The EU units directive permits it only for blood pressure and other body fluids, gives it as 133.322 pascals and allows no prefixes. The NHS treats 140/90 or higher at a clinic, or 135/85 at home, as high blood pressure, and 120 mmHg is 16.0 kilopascals. The capital H matters: Hg is the chemical symbol for mercury, while hg in lower case is a hectogram.