Standard Atmosphere

Standard Atmosphere (atm) is mainly used as a reference value for the average atmospheric pressure at sea level. It is often used to indicate the depth rating for a water resistant watch, but otherwise is rarely used as a unit for measuring pressure. 1 atm is defined as being exactly equal to 101,325 pascals.

Since the atmospheric pressure varies with changes in weather and altitude, it is convenient to standardise on a single value so that ratings, measurements and specifications can be compared. In particular 1 Standard Atmosphere is used by the Aviation industry as the reference standard for sea level pressure.