Pressure Converter

Convert between psi, bar, pascals, kilopascals, atmospheres and mercury column units.

Draft — figures checked, awaiting an editorial read. Not indexed.

Formula: Result = amount × (factor of source unit ÷ factor of target unit), with every factor expressed in pascals.

What this calculates

This converts pressure across the units used in tyres, weather reports, diving, plumbing and engineering. Pressure has an unusual number of competing units because so many different fields arrived at it independently — hence millimetres of mercury from medicine, bar from meteorology, psi from Anglo-American engineering, and the pascal from the SI system. All of them measure the same physical quantity: force per unit area.

How it works

The base is the pascal, which is one newton per square metre — a very small amount of pressure, which is why the kilopascal and the bar are more commonly used in practice. A bar is exactly 100,000 pascals and was chosen to be close to average atmospheric pressure at sea level, which is 101,325 pascals, defined as one standard atmosphere. Psi derives from pounds-force per square inch and works out to 6,894.757 pascals. The mercury units come from the height of a mercury column the pressure would support.

Worked example

A car tyre at 32 psi in bar: 32 × 6894.757 ÷ 100000 = 2.21 bar, which is why European tyre placards show around 2.2. In kilopascals, the same tyre is 220.6 kPa. A weather report of 1013 millibars is 101.3 kPa, or one standard atmosphere — normal sea-level pressure, which is exactly what that number is meant to convey.

Common mistakes

Confusing gauge pressure with absolute pressure. A tyre gauge reading 32 psi means 32 psi *above* atmospheric, so the absolute pressure inside is about 46.7 psi. Nearly every practical pressure measurement is gauge; most physics problems are absolute. Second, mixing bar and millibar, a factor of 1,000. Third, assuming psi and kPa are close because both are "small" numbers — they differ by a factor of about seven. Fourth, checking tyre pressure warm. Pressure rises with temperature by roughly 1 psi for every 10 °F, so a tyre checked after an hour of driving reads several psi high, and setting it to the placard figure then leaves it genuinely under-inflated once it cools. Manufacturer figures are always cold pressures.

Common questions

What is 32 psi in bar?

2.21 bar, or 220.6 kPa. Most European vehicles list tyre pressures in bar, and 2.2 is a common family car figure.

What is one atmosphere?

Standard atmospheric pressure at sea level: 101,325 pascals, which is 14.696 psi or 1.01325 bar.