Pressure is force spread over an area. The SI unit is the pascal, one newton per square metre, which is tiny: a sheet of paper resting on a table exerts about one pascal. That's why everyday pressures are usually quoted in larger units such as kilopascals, bar or pounds per square inch. Different fields have kept their own units, which is why the same idea appears on a weather map, a tyre gauge and a doctor's monitor in completely different numbers.
Weather: millibars and hectopascals
UK weather charts show air pressure in millibars, and the Met Office also uses hectopascals. They're the same: one millibar equals one hectopascal, 100 pascals. Average sea-level pressure is about 1,013 millibars. A deep Atlantic low bringing a storm might be below 960 millibars; a settled summer high might be above 1,030. A pressure of 990 millibars is 99 kilopascals, or about 14.36 psi.
Blood pressure: millimetres of mercury
Blood pressure is still measured in millimetres of mercury, mmHg, a unit from the days of mercury columns. A reading of 120/80 mmHg means 120 mmHg when the heart beats and 80 between beats. It's a gauge reading, measured above atmospheric pressure. In kilopascals that's about 16.0 and 10.7, but UK readings are almost always given in mmHg.
Gauge and absolute in practice
A tyre gauge reading 2.4 bar means 2.4 bar above the surrounding air. The absolute pressure inside the tyre is about 3.4 bar. Divers meet the same idea: water adds about one bar of pressure for every 10 metres of depth, so at 20 metres the absolute pressure is about three bar, three times the surface. That's why air in a diver's lungs and equipment behaves so differently at depth. Engineers write barg for gauge and bara for absolute to avoid confusion.
Worked conversions
A road bike tyre inflated to 80 psi is at about 5.5 bar. A car tyre recommended at 2.3 bar needs about 33 psi on a forecourt gauge. And a scuba cylinder filled to 232 bar holds air at about 3,365 psi, which is why cylinders must be tested regularly.
Checking your answer
Atmospheric pressure is a useful anchor: about 1 bar, 14.7 psi, 101 kPa or 1,013 millibars. Car tyres are typically two to three times that above atmospheric, around 2 to 2.5 bar or 30 to 36 psi. Bicycle tyres range from about 1.5 bar for mountain bikes to 7 bar or more for road bikes. If a converted tyre pressure is in hundreds of bar, a kilopascal figure has probably been read as bar.
Weather pressure varies over a much smaller range. Readings outside about 950 to 1,050 millibars are rare in the UK, so a converted forecast outside that range usually signals a slipped unit. Barometers in older houses may still be marked in inches of mercury, where about 29.9 inches corresponds to average sea-level pressure.
Pressure and altitude
Air pressure falls with height, roughly by about 12 millibars for every 100 metres near sea level. At the summit of Ben Nevis, 1,345 metres up, pressure is typically around 860 to 870 millibars, about 15% lower than at sea level. That's why a sealed crisp packet puffs up on a mountain walk, and why aircraft cabins, pressurised to the equivalent of a few thousand feet, make sealed bottles and packets expand.
One more worked example
A tyre pressure of 32 psi is about 2.2 bar or 221 kPa. If a European pump only shows bar and your handbook gives psi, set it to 2.2 bar. For the closest match, round to the pump's scale divisions and check the result with a separate gauge.