atmosphere
Pressureexact by definition
The standard atmosphere is defined as exactly 101 325 pascals. That value has been exact by definition since the 10th CGPM in 1954; before then it was defined as the pressure of a 760 mm column of mercury under standard gravity, which is where the number comes from. It is a reference pressure, not a measurement of the actual atmosphere on any given day.
Watch out: Real sea-level pressure varies by several percent with weather, and falls with altitude, so "one atmosphere" is a defined reference and not the pressure where you are standing. Note also the technical atmosphere (at), an older and different unit equal to 1 kgf/cm² or 98 066.5 Pa, about 3 percent smaller.
| 1 | 101.325 |
About the atmosphere
The standard atmosphere is a reference point rather than an observation. Fixing it at exactly 101 325 Pa in 1954 gave chemistry, aviation and materials testing a single agreed baseline so that "standard temperature and pressure" meant the same thing in every laboratory. The oddly specific number is inherited: it is the pressure of a 760 mm column of mercury of density 13 595.1 kg/m³ under standard gravity, which is what the original mercury-barometer definition produced.
Its most common role today is as the zero of gauge pressure. A tyre at 32 psi gauge is at about 3.2 atm absolute, because the atmosphere is already pressing on both sides of the gauge. It also anchors several other units: the torr is exactly 1/760 atm, and the millimetre of mercury is very nearly but not exactly the same thing. Note that IUPAC now recommends 100 kPa exactly, one bar, as the standard pressure for reporting thermodynamic data, so modern tables may be on a slightly different basis than older ones.
One atmosphere in every pressure unit
| 1,013,250 | microbar (μbar) | |
| 1,013,250 | barye (Ba) | |
| 760,000 | millitorr (mTorr) | |
| 101,325 | pascal (Pa) | convert → |
| 10,332.3 | millimeter of water column (mmH₂O) | |
| 2,116.22 | pound per square foot (lb/ft²) | |
| 1,033.23 | centimeter of water column (cmH₂O) | |
| 1,013.25 | hectopascal (hPa) | |
| 1,013.25 | millibar (mbar) | |
| 760 | torr (Torr) | |
| 760 | millimeter of mercury (mmHg) | convert → |
| 406.782 | inch of water column (in w.g.) | convert → |
| 101.325 | kilopascal (kPa) | convert → |
| 33.8985 | foot of water column (ft H₂O) | convert → |
| 29.9213 | inch of mercury (inHg) | convert → |
| 14.6959 | pound per square inch (psi) | convert → |
| 10.3323 | meter of water column (m H₂O) | |
| 10.1325 | decibar (dbar) | |
| 1.03323 | kilogram-force per square centimeter (kgf/cm²) | |
| 1.01325 | bar (bar) | convert → |
| 1 | atmosphere (atm)this unit | |
| 0.101325 | megapascal (MPa) | convert → |
| 0.0146959 | kip per square inch (ksi) | |
| 0.000101325 | gigapascal (GPa) | |
| 0.0000146959 | million pounds per square inch (Mpsi) |