Pressure Converter
Tyre pressure is gauge pressure, not absolute.
Tyre pressure is gauge pressure, not absolute. Pressure across pascals, bar, atmospheres, psi and mercury columns.
Converted
32 pounds per square inch
220,632.2334 pascals · 220.6322 kilopascals
One standard atmosphere is defined as exactly 101,325 pascals, and a bar is exactly 100,000 — which is why they are close but not equal. Tyre pressures are almost always gauge pressure, measured above atmospheric, so 32 psi in a tyre is about 46.7 psi in absolute terms.
How the Pressure Converter works
Pressure across pascals, bar, atmospheres, psi and mercury columns. One atmosphere is exactly 101,325 pascals and a bar is exactly 100,000 — close, and not equal.
Also known as: psi to bar converter · kpa to psi converter · tyre pressure converter · atmospheres to pascals
Gauge and absolute are different questions
Gauge pressure is measured relative to the surrounding atmosphere; absolute pressure includes it. A tyre at 32 psi gauge is at about 46.7 psi absolute, and a tyre reading zero is at atmospheric pressure rather than in a vacuum.
Almost every practical pressure reading is gauge, because gauges are open to the atmosphere on one side. Thermodynamic calculations need absolute, which is why the distinction has to be tracked deliberately.
Vacuum is the same idea from the other direction — pressure below atmospheric, quoted either as a negative gauge figure or as a small absolute one. A perfect vacuum is zero absolute and about −14.7 psi gauge at sea level.
Bar, atmosphere and why they are nearly equal
A standard atmosphere is defined as exactly 101,325 pascals; a bar is exactly 100,000. The 1.3% gap is small enough to ignore in a tyre and large enough to matter in precise work.
The bar was chosen as a round metric quantity close to atmospheric pressure, which is exactly why it is nearly but not quite equal. The millibar, a thousandth of a bar, is the meteorological unit and equals a hectopascal.
Millimetres of mercury survive in medicine because the original instrument was a mercury column. The unit is the height of mercury the pressure would support, and it persisted long after mercury manometers were replaced.
Pressure with depth and altitude
In water, pressure rises about one atmosphere every ten metres. At thirty metres a diver is at four atmospheres absolute, which is why air consumption rises so steeply with depth — each breath contains four times the gas at the surface.
In air, pressure falls roughly exponentially, halving at about 5,500 metres. That is why aircraft cabins are pressurised and why water boils at a lower temperature on a mountain.
The tyre-warming effect is the same physics at constant volume. Gas pressure rises with temperature, and tyres warm several degrees in normal driving — typically adding a few psi, which is why pressures are specified cold.
Where to go next
The Pressure Converter question rarely arrives on its own. These are the ones that usually come with it:
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Frequently asked questions
How do I convert psi to bar?
Divide by 14.504. Thirty-two psi is about 2.21 bar, which is a typical car tyre pressure.
Is one bar the same as one atmosphere?
Very nearly and not exactly. A standard atmosphere is 101,325 pascals and a bar is 100,000 — a 1.3% difference, which matters in precise work and not in a tyre.
What is the difference between gauge and absolute pressure?
Gauge pressure is measured above atmospheric; absolute includes it. A tyre at 32 psi gauge is at about 46.7 psi absolute, and a tyre at zero gauge is not a vacuum.
Why is blood pressure measured in mmHg?
Because the original instrument was a column of mercury, and the unit persisted after the instrument was replaced. It is the height of mercury the pressure would support.
What is a pascal?
One newton per square metre — the SI unit, and a very small amount of pressure. Atmospheric pressure is about 101,325 of them, which is why kilopascals are used in practice.
How does pressure change with altitude?
It falls roughly exponentially, dropping to about half at 5,500 metres. That is why aircraft cabins are pressurised and why water boils at a lower temperature on a mountain.
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