1atm

1 atm, or one atmosphere, is a unit of pressure commonly used in physics, chemistry, and engineering. It is defined exactly as 101,325 pascals (Pa). Historically, the unit was based on the pressure required to support a 760 millimeter column of mercury at 0 degrees Celsius in a barometer, linking it to the torr and to sea-level atmospheric pressure.

Conversions and equivalences are widely cited: 1 atm equals 101.325 kilopascals (kPa); about 14.696 pounds per square inch (psi); 760 torr; and roughly 1.01325 bar. In meteorology and some engineering contexts, pressure is often expressed in hectopascals (hPa), where 1 atm equals 1013.25 hPa.

Applications and usage: 1 atm is used to describe gas pressures, calibrate instruments, and compare laboratory conditions to Earth’s atmosphere. Although the pascal is the SI unit, the atm remains common in many fields because of historical usage and convenience in describing sea-level conditions. In many modern calculations, especially those requiring SI coherence, pressures are stated in pascals, kilopascals, or bars, but atm can still be found in legacy data, standard laboratory practices, and various industry standards.

Notes: Actual atmospheric pressure at sea level varies with weather and altitude, so 1 atm represents a standard reference value rather than a constant experienced everywhere at all times. The term standard atmosphere is related but refers to a broader atmospheric model used for calculations at sea level conditions.