PSI Converter (psi, atm, kgf/cm², torr)

Understanding the psi converter — concept and principle

This converter changes a pressure in pounds per square inch (psi) to and from bar, kilopascals, pascals, standard atmospheres, kgf/cm², torr and mmHg. Use it for car and bicycle tyres, air compressors, pressure washers, scuba tanks and hydraulic equipment, where American specifications use psi and European or Korean ones use bar, kPa or kgf/cm².

How it is calculated

The value is converted to pascals and then to the target unit:

result = value × (Pa per "from" unit) ÷ (Pa per "to" unit)

• 1 psi = 6,894.757293 Pa, so 1 psi = 0.0689475729 bar and 1 bar = 14.50377377 psi.
• 1 atm = 101,325 Pa = 14.69594878 psi.
• 1 kgf/cm² = 98,066.5 Pa = 14.22334331 psi.
• 1 torr = 101,325 ÷ 760 Pa.

The principle

A psi is the pressure of one pound-force (the weight of one avoirdupois pound, 0.45359237 kg, under standard gravity 9.80665 m/s²) spread over one square inch (0.0254² m²). Working that out gives 4.4482216 N ÷ 0.00064516 m² = 6,894.757 Pa. The kgf/cm² is built the same way from a kilogram-force on a square centimetre, which is why the two "engineering" units differ by a factor of about 14.22.

Worked example

Default inputs: 32 psi to bar.

1. Convert to pascals: 32 × 6,894.757293 = 220,632.2334 Pa.
2. Divide by 100,000 Pa per bar: 2.206322334 bar.
3. The same pressure is 220.6322334 kPa, 220,632.2334 ÷ 101,325 = 2.177470845 atm and 220,632.2334 ÷ 98,066.5 = 2.249822655 kgf/cm².

So a tyre at 32 psi is at about 2.2 bar or 2.25 kgf/cm².

How to read the result

Tyre and compressor gauges show gauge pressure (psig), the pressure above the surrounding atmosphere. Absolute pressure (psia) is about 14.7 psi higher at sea level. Passenger-car tyres are often set somewhere around 30–36 psi (about 2.1–2.5 bar), but always use the value on the vehicle's door-jamb placard or owner's manual, measured with cold tyres. The maximum pressure moulded on the tyre sidewall is a limit, not the recommended pressure.

Common mistakes

• Using the sidewall maximum as the inflation target.
• Treating 1 bar and 1 kgf/cm² as the same; they differ by about 2%.
• Adding atmospheric pressure to a tyre gauge reading when the specification is already in gauge pressure.
• Checking tyres right after driving, when warm air reads higher.

Frequently asked questions

How many psi is 1 bar?

14.50377377 psi.

What is 32 psi in bar?

About 2.21 bar.

How do I convert psi to kgf/cm²?

Divide by 14.2233. For example, 36 psi is about 2.53 kgf/cm².

What is the difference between psi, psig and psia?

psig is measured above atmospheric pressure, psia from a perfect vacuum; plain "psi" usually means psig on everyday gauges.

Sources: NIST Special Publication 811 (2008), Appendix B · BIPM, The International System of Units (SI Brochure), 9th edition