Water resistance (also known as "ATM rating") is one of the most important aspects to consider when buying a watch.
Watches are equipped with a water protection mechanism to prevent water from reaching delicate parts of the watch's body. The level of water resistance will vary from one watch to the next, but there is some standardization.
The ATM water resistance unit is used to measure such ratings. ATM is an acronym for "atmospheres." One ATM unit represents atmospheric pressure at sea level. People often misunderstand these "ATM" numbers and purchase watches that are not water-resistant.
An ATM Example
A reading of 5 ATM means that the watch can withstand five times the pressure at sea level (without sustaining water damage.) However,
this resistance references a fixed point, meaning movement at that depth is NOT considered.
What does that mean?
Movement and vibration can increase the forces on the watch (increasing the pressure). So in this example, if you were to swim at 5 ATMs, the pressure on the watch would end up being more than 5 ATMs.
Also important to note that this is a pressure measurement, not a depth measurement. Even though the pressure of 5 ATMs is approximately the same pressure as 50 meters (165 ft), it doesn't mean the watch can be used for diving at 50 meters.
ATM Cheat Sheet
Here is a breakdown of some typical watch ATM ratings. Always check with the watch manufacturer first and follow their guidance for water resistance.
0 ATM (Sea Level, 0 BAR, 0m, 0ft)
No water resistance.
1 ATM (1 BAR, 10m, 33ft)
A 1 ATM rating withstands pressures equivalent to a depth of 10 meters. Suitable for splashes or exposure to rain and snow.
3ATM (3 BAR, 30m, 100ft)
A 3 ATM rating withstands pressures equivalent to a depth of 30 meters. Suitable for splashes, rain and snow, showering, or jumping into the water (not diving).
5 ATM (5 BAR, 50m, 165ft)
A 5 ATM rating withstands pressures equivalent to a depth of 50 meters. Suitable for splashes, rain, snow, showering, diving into the water, swimming, and snorkeling.
10 ATM (10 BAR, 100m, 330ft)
A 10 ATM rating withstands pressures equivalent to a depth of 100 meters. Suitable for splashes, rain, snow, showering, diving into the water, swimming, snorkeling, and high-speed water sports (water skiing, jet skiing).
20 ATM (20 BAR, 200m, 660ft)
A 20 ATM rating withstands pressures equivalent to a depth of 200 meters, which is also known as the "diver's" rating. A 20 ATM watch is rated for skin diving and high-speed water sports.
Diver (up to the listed depth in meters)
"Diver" is an engineered rating (tested to the EN13319 standard) for dive compliance. Suitable for scuba diving and professional marine activity.
No matter what you read on the internet. ALWAYS check with the manufacturer before engaging in water activities that might damage your watch.