About the Pressure Converter
This pressure converter works across the seven units that turn up in engineering, medicine, meteorology and the garage: pascals, kilopascals, bar, pounds per square inch, standard atmospheres, torr and millimetres of mercury. Everything is referenced to the pascal, the SI unit, which is one newton spread across one square metre. That is a very small amount of pressure, which is why practical work is nearly always done in kilopascals or bar, and why atmospheric pressure comes out as the unwieldy figure of 101,325 pascals.
How to use the Pressure Converter
- 1
Enter the pressure reading in the Value box.
- 2
Select the unit it was measured in from the From list.
- 3
Select the unit you need from the To list.
- 4
Read the converted figure, and check the table to see the same pressure in all seven units at once.
What people use it for
Tyre pressures
European door-jamb stickers give bar while American gauges read psi. A 2.2 bar recommendation is 31.9 psi, and 220 kPa is the same pressure again.
Weather and altimetry
Charts use hectopascals, which are identical to millibars. A standard sea-level setting of 1013.25 hPa is one atmosphere, and a deep low might read 960 hPa.
Medical and laboratory readings
Blood pressure is reported in mmHg by universal convention. A reading of 120/80 mmHg corresponds to about 16.0/10.7 kPa.
The units and their exact relationships
The pascal is defined as one newton per square metre, and every other unit here is a fixed multiple of it. The bar is exactly 100,000 Pa, deliberately set close to average atmospheric pressure, which makes 1 millibar identical to 1 hectopascal — a coincidence meteorologists exploited when they switched notation without changing any numbers on their charts. The standard atmosphere is defined as exactly 101,325 Pa, equal to 1.01325 bar, 14.6959 psi or 760 mmHg. The pound per square inch derives from imperial force and area, giving 6,894.757 Pa, so one bar is 14.5038 psi. Torr and millimetres of mercury are almost but not quite the same: a torr is defined as exactly one seven-hundred-and-sixtieth of an atmosphere, which is 133.32237 Pa, while a millimetre of mercury is defined from the fluid column as 133.322387415 Pa. They differ by roughly one part in seven million, which is irrelevant clinically and matters only in high-precision vacuum work. Both descend from the mercury barometer, where pressure was read directly as the height of a supported column.
Gauge pressure, absolute pressure and why the distinction bites
Most everyday instruments read gauge pressure, meaning pressure relative to the surrounding atmosphere rather than relative to a vacuum. A tyre gauge showing 32 psi is telling you the air inside is 32 psi above the roughly 14.7 psi outside, so the absolute pressure is about 46.7 psi. The relationship is simply absolute equals gauge plus atmospheric, and the units are sometimes written psig and psia to keep them apart. Getting this wrong matters whenever gas laws are involved, because those equations require absolute pressure, and an engineer computing gas volume from a gauge reading will be adrift by a full atmosphere. It also explains why tyre pressure appears to drop in cold weather. Gas pressure falls with absolute temperature, and a 20 °C drop takes absolute temperature down by about 7 percent, which shows up as roughly 3 psi lost on a typical car tyre. Divers meet the same arithmetic from the other direction: every 10 metres of seawater adds about one atmosphere, so at 30 metres a diver is under roughly 4 atmospheres absolute, or about 405 kPa.
Tips
- One bar is 14.5 psi and roughly one atmosphere, so the three are interchangeable for rough work but not for calculation.
- Check tyre pressures cold; driving warms the air and can add 3 to 5 psi to the reading.
- Hectopascals and millibars are numerically identical, so a chart in either can be read without conversion.