Unit & Measurement Tables
Pressure Conversion Table
Pressure is force per area, and almost every conversion error involving it comes from the fact that three different quantities are in circulation at once. There is the SI unit, the pascal, which is one newton per square metre. There is the bar, a round 100,000 Pa adopted because a pascal is inconveniently small for weather and tyres. And there is the pound-force per square inch, which carries the pound as a force rather than as a mass and needs the standard acceleration of gravity to convert.
The distinction between psi and everything else is worth being precise about, because the pound appears twice in US engineering: as a mass of exactly 0.45359237 kg, and as a force of exactly 4.4482216152605 N. The bridge between them is the standard acceleration of gravity, 9.80665 m/s², which is itself an exact conventional value. A psi is that force over an inch squared, so it is exact to as many decimals as you care to carry.
The third table gives the exact factors, and the practical takeaway from it is that a bar and an atmosphere are not the same thing. They differ by 1.3%, which is small enough to be invisible on a tyre gauge and large enough to matter in a calibration or a gas law calculation.
Pounds per square inch to bar, kPa, atmospheres and mmHg
| psi | bar | kPa | atm | mmHg |
|---|---|---|---|---|
| 5 | 0.345 | 34.47 | 0.3402 | 258.6 |
| 10 | 0.689 | 68.95 | 0.6805 | 517.1 |
| 15 | 1.034 | 103.42 | 1.0207 | 775.7 |
| 20 | 1.379 | 137.90 | 1.3609 | 1,034.3 |
| 25 | 1.724 | 172.37 | 1.7011 | 1,292.9 |
| 30 | 2.068 | 206.84 | 2.0414 | 1,551.4 |
| 32 | 2.206 | 220.63 | 2.1775 | 1,654.9 |
| 35 | 2.413 | 241.32 | 2.3816 | 1,810.0 |
| 40 | 2.758 | 275.79 | 2.7218 | 2,068.6 |
| 50 | 3.447 | 344.74 | 3.4023 | 2,585.7 |
| 60 | 4.137 | 413.69 | 4.0828 | 3,102.9 |
| 80 | 5.516 | 551.58 | 5.4437 | 4,137.2 |
| 100 | 6.895 | 689.48 | 6.8046 | 5,171.5 |
| 120 | 8.274 | 827.37 | 8.1655 | 6,205.8 |
| 150 | 10.342 | 1,034.21 | 10.2069 | 7,757.2 |
| 200 | 13.790 | 1,378.95 | 13.6092 | 10,343.0 |
A typical car tyre is around 32 psi, which is 2.206 bar or 220.63 kPa. The bar is close to an atmosphere but not the same: 1 bar is 100 kPa exactly and 1 atm is 101.325 kPa.
Bar to psi, kPa, atmospheres and mmHg
| bar | psi | kPa | atm | mmHg |
|---|---|---|---|---|
| 0.5 | 7.25 | 50.00 | 0.4935 | 375.0 |
| 1 | 14.50 | 100.00 | 0.9869 | 750.1 |
| 1.5 | 21.76 | 150.00 | 1.4804 | 1,125.1 |
| 2 | 29.01 | 200.00 | 1.9738 | 1,500.1 |
| 2.5 | 36.26 | 250.00 | 2.4673 | 1,875.2 |
| 3 | 43.51 | 300.00 | 2.9608 | 2,250.2 |
| 3.5 | 50.76 | 350.00 | 3.4542 | 2,625.2 |
| 4 | 58.02 | 400.00 | 3.9477 | 3,000.2 |
| 5 | 72.52 | 500.00 | 4.9346 | 3,750.3 |
| 6 | 87.02 | 600.00 | 5.9215 | 4,500.4 |
| 8 | 116.03 | 800.00 | 7.8954 | 6,000.5 |
| 10 | 145.04 | 1,000.00 | 9.8692 | 7,500.6 |
Bar is a round 100,000 Pa, which is why it appears on European gauges and in weather reporting. One bar is 14.5038 psi.
Exact conversion factors
| Unit | In pascals | In kPa | In psi |
|---|---|---|---|
| 1 psi | 6,894.757293 | 6.894757 | 1 |
| 1 bar | 100,000 | 100.000 | 14.503774 |
| 1 atm | 101,325 | 101.325 | 14.695949 |
| 1 mmHg | 133.322387 | 0.133322 | 0.019337 |
| 1 inHg | 133.322387 | 0.133322 | 0.019337 |
The millimetre of mercury is a conventional value rather than a defined one, fixed at 133.322387415 Pa. Blood pressure cuffs and barometers both use it, which is why a hospital reading and a weather report share a unit.
How to Use These Tables
- Tyre pressures: psi to bar is × 0.0689476, and bar to psi is × 14.5038. A 2.4 bar specification is about 35 psi.
- Weather: standard atmospheric pressure is 101.325 kPa, 1.01325 bar or 29.92 inHg. Readings below that usually indicate a low pressure system.
- Gas law work: convert everything to pascals before using PV = nRT, since the gas constant is quoted in SI. One atmosphere is 101,325 Pa.
- Gauge versus absolute: a gauge reads pressure above atmospheric, so a tyre at 32 psi gauge holds about 46.7 psi absolute at sea level. Most formulas want absolute pressure.
Common Questions
Is a bar the same as an atmosphere?
No. A bar is exactly 100,000 Pa and an atmosphere is exactly 101,325 Pa, so a bar is 1.3% smaller. Tyre gauges and weather reports use bar; chemistry and aviation still use atmospheres.
What does the psi in a tyre pressure actually mean?
Pounds-force per square inch. The pound is being used as a force, equal to 4.4482216152605 N, which is a mass of 0.45359237 kg under standard gravity. That is why psi has no simple decimal relationship to metric pressure units.
What is the difference between gauge and absolute pressure?
Gauge pressure is measured relative to ambient air; absolute pressure is measured relative to a vacuum. They differ by about 14.7 psi or 101.325 kPa at sea level, and gas law calculations require the absolute figure.
Why is blood pressure still measured in mmHg?
Because the original instrument was a column of mercury, and the unit stuck. It is now defined conventionally as 133.322387415 Pa, so a reading of 120 mmHg is about 16 kPa.
How do I convert kPa to psi quickly?
Divide by 6.89476. For a rough figure, 1 psi is very close to 6.9 kPa, so 200 kPa is about 29 psi. Using 7 instead introduces about 1.5% error, which is fine for a tyre and not for a calibration.
More Unit & Measurement Tables
Sources
- NIST Special Publication 811 — pressure units and the pascal — National Institute of Standards and Technology
- Standard atmosphere and pressure unit definitions — National Institute of Standards and Technology
- SI Brochure — definitions of the pascal and the bar — International Bureau of Weights and Measures
Filed with Unit Conversion calculations.