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

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

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.

pascal | bar | standard atmosphere | torr | pound per square inch | |
---|---|---|---|---|---|

Pa | bar | atm | Torr | psi | |

1 Pa | ≡ 1 N/m^{2} |
10^{−5} |
9.8692×10^{−6} |
7.5006×10^{−3} |
145.04×10^{−6} |

1 bar | 10^{5} |
≡ 10^{5} Pa |
0.98692 | 750.06 | 14.5037744 |

1 at | 0.980665 ×10^{5} |
0.980665 | 0.96784 | 735.56 | 14.223 |

1 atm | 1.01325 ×10^{5} |
1.01325 | ≡ p_{0} |
760 | 14.696 |

1 Torr | 133.322 | 1.3332×10^{−3} |
1.3158×10^{−3} |
≈ 1 mm_{Hg} |
19.337×10^{−3} |

1 psi | 6.895×10^{3} |
68.948×10^{−3} |
68.046×10^{−3} |
51.715 | ≡ 1 lbf/in^{2} |