Understanding pressure
Force per unit area.
The pascal is the SI unit. The bar, the psi, and the atmosphere are everyday survivors.
The pascal and the bar.
A pascal is one newton per square metre — a tiny pressure, about the weight of a thin sheet of paper resting on a table. So everyday pressures are usually given in kilopascals or megapascals, or in the cousin unit, the bar (exactly 100 000 Pa). Atmospheric pressure at sea level is roughly one bar, by design.
1 bar ≡ 100 000 Pa · 1 atm ≡ 101 325 Pa
PSI, atmospheres, mmHg.
Each region and trade has a favourite. Tyres in the United States are read in pounds per square inch; weather and aviation use atmospheres or hectopascals; medicine still measures blood pressure in millimetres of mercury, a holdover from the mercury-column manometer.
Practical equivalences
- 1 psi ≈ 6894.757 Pa ≈ 0.0689 bar
- 1 atm = 1.01325 bar = 14.696 psi
- 1 atm = 760 mmHg
- 1 hPa = 100 Pa = 1 mbar (weather forecast)
Three worked conversions.
32 psi to bar
1 psi ≈ 6894.757 Pa · 1 bar = 100 000 Pa
Convert through pascals.
32 × 6894.757 ÷ 100 000 ≈ 2.206
= 2.206 bar
120 mmHg to kPa
1 mmHg ≈ 133.322 Pa
Multiply by the factor and divide for kPa.
120 × 133.322 ÷ 1000 ≈ 15.999
= 15.999 kPa
1 atm to psi
1 atm = 14.696 psi
Direct equivalence.
1 × 14.696 = 14.696
= 14.696 psi
Gauge vs absolute.
Tyre gauges and most industrial dials read gauge pressure — pressure above the surrounding atmosphere. Absolute pressure adds the local atmospheric pressure back in. A tyre at 32 psi gauge is at about 46.7 psi absolute. The converter treats all values as the same kind; if you're working between gauge and absolute, do the addition by hand.
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