The psi to bar formula
Multiply the pressure in pounds per square inch by 0.0689475729 to get bar. 32 psi is 32 × 0.0689475729 = 2.2063 bar. To go the other way multiply bar by 14.5037738.
The factor comes from the definitions. A psi is one pound-force on one square inch. A pound is exactly 0.45359237 kg, standard gravity is exactly 9.80665 m/s² and an inch is exactly 0.0254 m, so one psi is 0.45359237 × 9.80665 ÷ 0.0254², which is about 6,894.757293 Pa, and a bar is exactly 100,000 Pa. Because bar is a round number of pascals and psi is not, the conversion never comes out as a short decimal. For a quick estimate, divide psi by 14.5 and you are within 0.03%.
Tyre pressure
Cars made for the US market list tyre pressure in psi and cars sold in much of Europe list it in bar or kPa, so the two units meet at every tyre shop and on every rental. Many passenger cars call for something around 30 to 36 psi, which is about 2.1 to 2.5 bar, but the figure that counts is the one on the placard in the door jamb or in the handbook. The number molded into the tyre sidewall is the most the tyre can take, not the pressure to use.
Check tyres when they are cold. Driving warms the air inside and raises the reading. If your compressor is in psi and your placard is in bar, press the swap button after entering the bar figure, or look for the nearest bar figure in the table below the converter and read the psi beside it.
Tanks, washers and pipes
- Scuba: a US cylinder filled to 3,000 psi holds 206.8 bar. European cylinders are often filled to 200 bar, which is 2,900.8 psi.
- Pressure washers: a machine rated at 2,000 psi delivers 137.9 bar.
- Household water: many homes sit between 40 and 80 psi, which is 2.8 to 5.5 bar.
- Atmosphere: 14.7 psi is the rounded figure for one atmosphere. It is 1.0135 bar, a little more than the standard atmosphere of 1.01325 bar, because 14.7 is itself rounded up from 14.696.
These are well known figures that vary by equipment, so check the rating on the equipment you use.
psi is not always gauge pressure, and bar is not always absolute
Everyday psi and bar readings, like those on a tyre gauge, are gauge pressure. Engineers write psig for gauge pressure and psia for absolute pressure. Weather and physics data are usually absolute. This converter changes only the unit, so a gauge value stays a gauge value. Add about 14.7 psi, or 1.013 bar, if you need the absolute figure at sea level.