Absolute, Gage, Vacuum, and Atmospheric Pressures |

P_{gage} = P_{abs} − P_{atm}
| gage pressure |

P_{vac} = P_{atm} − P_{abs}
| vacuum pressure |

P_{abs} = P_{atm} + P_{gage}
| absolute pressure |

P_{abs} | absolute pressure |

P_{gage} | gage pressure |

P_{vac} | vacuum pressure |

P_{atm} | atmospheric pressure |

The atmospheric pressure is the pressure that an area experiences due to the force exerted by the atmosphere. For engineering calculations typically the pressure used is the pressure at sea level. Typically, the quantity used for engineering calculations is 1 atm, or 101 kPa. Gage pressure is the pressure relative to the atmospheric pressure. In other words, how much above or below is the pressure with respect to the atmospheric pressure. Absolute pressure is the sum of the atmospheric pressure and the gage pressure. If the gage pressure has a positive value, the absolute pressure will be greater than the atmospheric pressure. If the gage pressure has a negative value, the absolute pressure will be less than the atmospheric pressure. Absolute pressure can be abbreviated by

The actual pressure at a given position is called the absolute pressure, and it is measured relative to absolute vacuum (i.e., absolute zero pressure). Most pressure-measuring devices, however, are calibrated to read zero in the atmosphere, and so they indicate the difference between the absolute pressure and the local atmospheric pressure. This difference is called the gage pressure. Pressures below atmospheric pressure are called vacuum pressures and are measured by vacuum gages that indicate the difference between the atmospheric pressure and the absolute pressure.

Like other pressure gages, the gage used to measure the air pressure in an automobile tire reads the gage pressure. Therefore, the common reading of 32 psi (2.25 kgf/cm

With respect to thermodynamic relations and tables, absolute pressure is almost always used. Often the letters "a" (for absolute pressure) and "g" (for gage pressure) are added to pressure units (such as psia and psig) to clarify what is meant.