Convert between pascal, bar, psi, atm and mmHg instantly.
Pressure is force per unit area, and it has an unusually large number of units in active use because meteorology, tyre inflation, medicine, vacuum work and engineering each settled on their own. This tool converts between all the common ones through the pascal, the SI unit, which is one newton per square metre.
The pascal is a very small unit — atmospheric pressure is about 101,325 of them — so it is almost always used with a prefix. Kilopascals and hectopascals dominate in practice.
| Unit | In pascals | Where used |
|---|---|---|
| Pascal (Pa) | 1 | SI base; scientific work |
| Kilopascal (kPa) | 1,000 | Tyre pressure outside the US, engineering |
| Hectopascal (hPa) | 100 | Meteorology (identical to the millibar) |
| Bar | 100,000 | Industry, diving, European tyre pressure |
| Millibar (mbar) | 100 | Weather charts |
| Atmosphere (atm) | 101,325 | Chemistry reference conditions |
| psi | 6,894.76 | US tyre and industrial pressure |
| mmHg / torr | 133.322 | Blood pressure, vacuum |
| inHg | 3,386.39 | US aviation altimetry, barometers |
Note that the hectopascal and the millibar are numerically identical, which is why weather reports switched from millibars to hectopascals without any of the numbers changing. Standard sea level pressure is 1013.25 in both.
This distinction causes more real errors than any conversion factor. Gauge pressure is measured relative to the surrounding atmosphere; absolute pressure is measured from a perfect vacuum.
A tyre gauge reading 32 psi is reporting 32 psi above atmospheric, so the absolute pressure inside is about 46.7 psi. A completely flat tyre reads 0 on the gauge but still contains air at roughly 14.7 psi absolute. Units are sometimes disambiguated as psig for gauge and psia for absolute, and mixing them in a calculation involving the gas laws — which require absolute pressure — produces badly wrong answers.
Common conversions for the figures on a door pillar placard:
| psi | kPa | bar |
|---|---|---|
| 28 | 193 | 1.93 |
| 32 | 221 | 2.21 |
| 35 | 241 | 2.41 |
| 40 | 276 | 2.76 |
Two practical notes. Use the figure on the vehicle's placard, not the maximum moulded into the tyre sidewall — the latter is a limit, not a recommendation. And check pressures cold, before driving: tyres warm up in use and gain roughly 1 psi for every 10 F rise, so a hot reading overstates the cold pressure by several psi. Ambient temperature has the same effect, which is why tyres appear underinflated on the first cold morning of winter.
Blood pressure is reported in millimetres of mercury, a unit dating from mercury column manometers. A reading of 120/80 mmHg means systolic pressure of 120 and diastolic of 80, both gauge pressures relative to atmospheric. In SI terms 120 mmHg is about 16 kPa. Medicine has retained mmHg despite the metric transition because the entire clinical literature and every reference threshold is expressed in it, and renumbering would introduce more risk than it removed. The torr is essentially the same unit under a different name, used in vacuum work.
Divide by 14.5038, or multiply by 0.0689. So 32 psi is about 2.21 bar. Going the other way, multiply bar by 14.5 — 2 bar is 29 psi.
Yes, exactly. One hectopascal equals one millibar, which is why meteorology could switch names without changing any numbers. Standard sea level pressure is 1013.25 in either unit.
Gauge pressure is measured relative to the surrounding atmosphere; absolute pressure is measured from a vacuum. Absolute equals gauge plus about 14.7 psi at sea level. Tyre gauges and blood pressure readings are gauge; gas law calculations need absolute.
No. The sidewall figure is the tyre's maximum safe pressure, not the recommendation for your vehicle. Use the placard on the driver's door pillar or in the manual, and check when the tyres are cold.
Because the unit comes from mercury manometers and every clinical threshold, guideline and study in the literature is expressed in it. Converting the whole body of medical reference material would create more risk than the metric consistency would be worth.