Pressure Converter

Gauge versus absolute is the critical distinction

Gauge pressure is measured relative to the surrounding atmosphere; absolute pressure is measured from a vacuum. They differ by about one atmosphere, 14.7 psi or 1.013 bar. A tyre inflated to 32 psi holds 32 psi gauge, which is roughly 46.7 psi absolute. Converting a gauge reading as though it were absolute, or the reverse, produces an error of a full atmosphere, and in engineering contexts that is the difference between a correct calculation and a dangerous one. Units are often written psig and psia to disambiguate, and where they are not, ask.

The SI unit is inconveniently small

One pascal is one newton per square metre, which is a tiny amount of pressure: atmospheric pressure is about 101,325 Pa. That is why practical work uses kilopascals, bar or psi. A bar is exactly 100,000 Pa, chosen to be close to atmospheric pressure while remaining a round metric figure.

Mercury columns persist in two fields

Blood pressure is universally reported in millimetres of mercury, and weather in inches of mercury in the United States. Both descend from actual mercury barometers, and both survive because the numbers are familiar to their practitioners rather than because the unit is convenient. Standard atmospheric pressure is 760 mmHg or 29.92 inHg.

Tyre pressure and temperature

Pressure rises with temperature in a sealed volume, roughly 1 psi for every 10 degrees Fahrenheit. That is why tyres should be checked cold: driving warms them and a reading taken after a journey can be 4 to 6 psi higher than the true cold pressure. Setting pressure from a warm reading leaves the tyres genuinely underinflated.

Altitude changes the baseline

Atmospheric pressure falls with altitude, roughly halving every 5,500 metres. Any gauge reading therefore depends on where you are standing, which is another reason absolute pressure is what engineering calculations use.

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