A watch's water-resistance is measured in bars (a unit of pressure where 1 bar equals 1 atmosphere or atm).
Manufacturers generally indicate this water-resistance in metres (m), feet (ft), or atmospheres (atm).
A watch that is described as water-resistant, with or without an additional indication of overpressure, must be conform and tested to the criteria set out in NIHS 92-10 (equivalent to ISO-2281 international standard). These watches are destined for ordinary everyday use, including periods of immersion in water such as leisure swimming. They can be used in conditions of changing air pressure, water pressure or temperature. However, even with an overpressure indication they are not intended to be worn for underwater diving.
A diving watch is made to be worn underwater at a depth of at least 100 metres (330 feet). It must include a time control device and fully conform to the criteria set out in NIHS 92-11 standard (ISO 6425) in terms of luminosity, shock-resistance, anti-magnetism and the solidity of the strap.