How to convert pressure units
Pressure is force per area, and every unit converts through the pascal (one newton per square metre). This table shows what each unit is worth in psi and bar:
| Unit | In psi | In bar |
|---|---|---|
| 1 psi | 1 | 0.0689476 |
| 1 bar | 14.5038 | 1 |
| 1 kPa | 0.145038 | 0.01 |
| 1 MPa | 145.038 | 10 |
| 1 atm | 14.6959 | 1.01325 |
| 1 inHg | 0.491154 | 0.0338639 |
| 1 kgf/cm² | 14.2233 | 0.980665 |
Definitions used: 1 psi = 6,894.757293168 Pa, 1 bar = 100,000 Pa, 1 atm = 101,325 Pa, 1 inHg = 3,386.389 Pa, 1 mmHg = 133.322387415 Pa, 1 mH₂O = 9,806.65 Pa and 1 kgf/cm² = 98,066.5 Pa.
Worked example
The calculator starts at 100 psi, a typical plant air pressure:
- In bar: 100 × 0.0689476 = 6.89476 bar
- In kPa: 100 × 6.89476 = 689.476 kPa
- In kgf/cm²: 7.0307 kgf/cm²
The chart above lists common values from psi to bar, bar to psi and inHg to psi, ready to read.
Gauge vs absolute pressure
A gauge reads gauge pressure: how far above the atmosphere you are. Tires, compressors and air lines always talk gauge. Absolute pressure counts from a vacuum and is what you need for vacuum work, gas laws and steam tables. Unit conversion works the same for both, but never mix them: 100 psig is about 114.7 psia at sea level.
Common mistakes
- Mixing psig and psia when comparing a datasheet with a gauge.
- Reading MPa as bar. 1 MPa = 10 bar = 145 psi. Hydraulics often come in MPa.
- Treating kgf/cm² as bar in calibration work. They differ by 2%.
For bolt torque, use the torque converter, and for pumps and compressors the flow rate converter.