Pressure Converter
Pressure Converter
How pressure conversion works
Pressure is force divided by area, and the SI unit, the pascal, is one newton per square metre. That is a very small amount of pressure, so practical units are all large multiples of it: a bar is 100,000 pascals, and one standard atmosphere is 101,325 pascals exactly.
The trap in pressure figures is not the arithmetic. It is that most everyday pressure readings are gauge pressure, measured relative to the surrounding air, while physics and chemistry use absolute pressure, measured relative to a vacuum. The two differ by one atmosphere, which is about 14.7 psi.
Formulas
Worked examples
| From | To | Result |
|---|---|---|
| 1 bar | psi | 14.5038 psi |
| 32 psi | bar | 2.20632 bar |
| 1 atm | kPa | 101.325 kPa |
| 760 mmHg | kPa | 101.325 kPa |
| 100 kPa | psi | 14.5038 psi |
Results are computed with the same factors the converter above uses, rounded to six significant figures.
Common pressures
| psi | bar | kPa |
|---|---|---|
| 14.696 | 1.01325 | 101.325 |
| 29 | 1.99948 | 199.948 |
| 32 | 2.20632 | 220.632 |
| 35 | 2.41316 | 241.316 |
| 100 | 6.89476 | 689.476 |
| 145 | 9.9974 | 999.74 |
The first row is one standard atmosphere. 29–35 psi is the normal range for a passenger car tyre.
Worth knowing
- 1 bar = 100,000 Pa exactly. It is not an SI unit but is accepted for use with SI, and it was chosen to be close to one atmosphere.
- 1 standard atmosphere = 101,325 Pa exactly, by definition. It is close to but not the same as 1 bar, which is 0.9869 atm.
- 1 psi ≈ 6,894.76 Pa. The name expands to pounds per square inch.
- Car tyre pressures are gauge pressure. A tyre at "32 psi" contains air at about 46.7 psi absolute; a completely flat tyre reads 0 psi gauge and 14.7 psi absolute.
Frequently asked questions
- How many psi is 1 bar?
- 14.5038 psi. The two are often treated as interchangeable in tyre pressure contexts because 2 bar and 29 psi are close, but the 3% gap adds up at higher pressures.
- What is the difference between gauge and absolute pressure?
- Gauge pressure is measured relative to the surrounding atmosphere; absolute pressure is measured relative to a vacuum. They differ by one atmosphere, about 14.7 psi. Tyre gauges, blood pressure cuffs and boiler gauges all read gauge pressure, so a reading of zero means "same as the air outside", not "no pressure at all".
- Why is one atmosphere 101,325 pascals exactly?
- Because it was defined that way in 1954, chosen to match the pressure of 760 millimetres of mercury at 0 °C under standard gravity. It is a definition rather than a measurement, so it carries no uncertainty. Actual sea level pressure varies by several percent with weather.
- What pressure should my car tyres be?
- Whatever the placard in the driver's door jamb or the fuel filler flap says, not what is written on the tyre sidewall: the sidewall figure is the maximum the tyre can take, not the recommended pressure for your vehicle. Most passenger cars sit between 29 and 35 psi, or roughly 2.0 to 2.4 bar.