Atm (atmosphere) is a term used to measure the atmospheric pressure of a watch case. The atmospheric pressure is measured in units of atmospheres (atm) or bars (bar) and is used as a measure of waterproofness in watches. A watch with a rating of 3 atm is suitable for everyday use, including swimming and showering. A watch with a rating of 10 atm is suitable for deep-water activities such as scuba diving.
