Three units, one quantity, and a fourth complication that matters more than any of them: whether the number you are reading is measured from vacuum or from the air around you.
Get the unit conversion wrong and you are out by a factor. Get the gauge-versus-absolute distinction wrong and you are out by a whole atmosphere — about 14.7 psi — which on a tyre is catastrophic and on a process vessel is worse.
The three units
| Unit | Definition | SI? | Where you meet it |
|---|---|---|---|
| Pascal (Pa) | 1 newton per square metre | Yes — the SI derived unit | Standards, engineering, weather (as hPa) |
| Bar | Exactly 100,000 Pa | No, but accepted for use | European tyres, diving, hydraulics, espresso |
| psi | 1 pound-force per square inch = 6,894.757 Pa | No | US and UK tyres, compressed air, plumbing |
The conversions worth committing to memory:
- 1 bar = 14.504 psi = 100 kPa
- 1 psi = 0.0689 bar = 6.895 kPa
- 1 standard atmosphere = 1.01325 bar = 14.696 psi = 101.325 kPa
Note the third line. A bar is close to an atmosphere but 1.3% below it. That was the design intent — the bar was chosen to be a round metric number near atmospheric pressure — and it is close enough that people treat them as equal, which is fine for a tyre and not for a calibration.
Gauge versus absolute
A tyre gauge reads zero when it is lying on the bench, even though the air around it is at about 101 kPa. It reads pressure relative to ambient. That is gauge pressure.
Absolute pressure is measured from a perfect vacuum. The relationship is simply:
absolute = gauge + atmospheric — so a tyre at 32 psi gauge is at about 46.7 psi absolute.
Notations you will see:
- psig / bar g / kPa (g) — gauge
- psia / bara / kPa (a) — absolute
- Unmarked — usually gauge in workshop contexts, usually absolute in thermodynamics and process engineering
That last line is the dangerous one. An unlabelled figure means opposite things in a tyre manual and a steam table. When a specification matters, find the label; if there isn't one, find out which convention the document uses before you convert anything.
Tyre pressure, done properly
Tyres are where most people meet this conversion, so it is worth being precise about a few things.
| psi | bar | kPa |
|---|---|---|
| 26 | 1.79 | 179 |
| 30 | 2.07 | 207 |
| 32 | 2.21 | 221 |
| 35 | 2.41 | 241 |
| 40 | 2.76 | 276 |
| 44 | 3.03 | 303 |
- The sidewall number is a maximum, not a recommendation. The correct cold inflation pressure comes from the placard in the door jamb, the fuel filler flap, or the handbook.
- "Cold" means cold. Driving heats the air; pressure rises roughly 1 psi per 10 °F (about 0.07 bar per 5.5 °C) of temperature increase. Check before driving, or add an allowance.
- Ambient temperature matters too. The same tyre loses about 1 psi for every 10 °F drop in outside temperature, which is why warning lights appear on the first cold morning of autumn.
- Placards round differently in each unit. A car specified at 2.2 bar in Europe and 32 psi in the US is the same recommendation — 2.2 bar is 31.9 psi. Neither is more correct.
Other places these units turn up
- Diving. Cylinders are 200 or 300 bar in most of the world, 3,000 psi in the US. 200 bar is 2,901 psi, so a "3,000 psi" and a "200 bar" cylinder are not the same fill.
- Espresso. The traditional 9 bar brew pressure is 130.5 psi. Machines advertising "15 bar" are quoting pump capability, not brew pressure.
- Weather. Reported in hectopascals; 1,013.25 hPa is standard sea level. A hectopascal is exactly one millibar, so the two numbers are always identical.
- Materials. Strengths are in megapascals. 1 MPa = 145 psi, so a 400 MPa steel is about 58,000 psi. Watch the kPa/MPa factor of 1,000 here — it is the easiest three-orders-of-magnitude slip on the list.
- Blood pressure. Reported in mmHg, a fourth unit entirely. 120 mmHg is 16 kPa or 2.3 psi. It is a medical convention that has resisted metrication.
Precision and safety
The conversion factors are exact — they follow from the exact definitions of the pound-force and the inch. Your gauge is not: a typical tyre gauge is accurate to about ±1 psi, which is ±0.07 bar. Quoting a converted tyre pressure to two decimal places in bar implies an instrument nobody in a garage owns.
One rounding rule worth stating plainly: on anything pressure-rated for safety — cylinders, hoses, relief valves, vessels — do not round a converted figure toward the more convenient number. Use the manufacturer's stated limit in its original unit, and treat the converted value as a reading aid rather than as the specification.