Units of Measurement/Pressure

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This figure shows pressure exerted by particle collisions inside a closed container. The collisions that exert the pressure are highlighted in red.

Pressure is the force per unit area applied in the direction perpendicular to a surface. Mathematically, pressure is defined as

 P = \frac{dF}{dA}

where:

P is pressure
F is the component of force perpendicular to the surface
A is the area of the surface

When a force is constant over an area, the pressure acting on that area is simply

 P = \frac{F}{A}

Pressure is a scalar quantity, thus it acts in all directions at any given point. In order for pressure to create a force, the pressure must be integrated over some area.

Pressure units
pascal bar standard atmosphere torr pound per square inch
Pa bar atm Torr psi
1 Pa ≡ 1 N/m2 10−5 9.8692×10−6 7.5006×10−3 145.04×10−6
1 bar 105 ≡ 105 Pa 0.98692 750.06 14.5037744
1 at 0.980665 ×105 0.980665 0.96784 735.56 14.223
1 atm 1.01325 ×105 1.01325 p0 760 14.696
1 Torr 133.322 1.3332×10−3 1.3158×10−3 ≈ 1 mmHg 19.337×10−3
1 psi 6.895×103 68.948×10−3 68.046×10−3 51.715 ≡ 1 lbf/in2