ATM and Bar
ATM and bar are the pressure units used for water resistance. One ATM equals about one bar, roughly the static pressure of 10 meters of water, so a 20 ATM watch is rated to 200m. That figure is a laboratory pressure rating, not an actual diving depth.
At a glance
- Unit relationship
- 1 ATM is about 1 bar and roughly the static pressure of 10 meters of water
- Example value
- A 20 ATM watch is marked as 200 meters
- Key distinction
- It is a laboratory pressure rating, not a real diving depth
When you see a figure like 10 ATM or 20 bar printed on a case, what the maker is really stating is the pressure the case was tested to hold. Converting that pressure into meters is the familiar shorthand, but knowing how the conversion works is the key to reading the number correctly.
How the units convert
Three values say the same thing in different language:
- ATM (atmosphere): the base unit of pressure; 1 ATM is about 1 bar
- bar: very nearly identical to ATM, and often printed on case backs in its place
- meters: each ATM equals roughly the static pressure of 10 meters of water, so 20 ATM is written as 200 meters
That is why 5 ATM, 5 bar and 50 meters all describe the same resistance class.
Why the meters are not a real depth
The meter figure here is a fixed laboratory pressure, not a diving depth. In real use, swimming motions, splashing water and temperature swings load the case with brief surges above the marked figure. In practice a watch's relationship with water is set by its water resistance class, and serious diving asks for dive watch standards on top of it. For real examples, see our guide to the best dive watches under $500.
Examples
If you read 20 ATM on a case back, that is the same as 20 bar and 200 meters: the pressure class the case was tested to withstand.
View this watch
Comparison
Two figures printed side by side on the same case carry the same information, yet one of them is routinely misread.
| Option A | Option B | Notes |
|---|---|---|
| ATM and bar (pressure) | Meters (looks like a depth) | ATM and bar measure pressure directly and are very nearly the same as each other. The meters beside them are a conversion of that pressure, roughly 10 meters per ATM, and stand for a fixed laboratory pressure rather than a real diving depth. |
Related terms
Watches that show this
Frequently asked questions
How many meters is 20 ATM?
20 ATM is marked as 200 meters. Since each ATM equals roughly the static pressure of 10 meters of water, 20 ATM works out to 200 meters and to about 20 bar. That is the pressure the case was tested to, not a real diving depth.
Is there a difference between ATM and bar?
In practice it is negligible. One ATM equals about one bar, so the two are used interchangeably on water resistance labels. Some makers print ATM on the case back and others print bar, and both describe the same pressure class.
Can I dive to 200 meters with a watch rated 200 meters?
No, and that figure should not be read that way. The 200 meters on the case is a fixed laboratory pressure, not a real diving depth. In real use, motion and splashing add brief surges of load, so you should not treat the marked figure as a usable depth.