A watch rated to 100 meters should be fine for scuba diving, right? Wrong. Water resistance ratings are one of the most misunderstood specifications in watchmaking. Here's what those numbers actually mean.
The Rating Doesn't Mean What You Think
Water resistance ratings like "50m" or "100m" refer to static pressure tests in a laboratory, not real-world diving depths. A 50-meter rating doesn't mean you can dive to 50 meters—it means the watch can withstand the equivalent static pressure of 50 meters of water, briefly, in a controlled test.
In real life, movement creates dynamic pressure. Jumping into a pool, swimming strokes, or water from a showerhead can generate pressure spikes far exceeding the static rating. That's why the guidelines for actual use are much more conservative than the numbers suggest.
Water Resistance Reality Guide
- 30m / 3 ATM: Splash resistant only—no swimming, no shower
- 50m / 5 ATM: Brief swimming okay, no diving or water sports
- 100m / 10 ATM: Swimming and snorkeling safe
- 200m / 20 ATM: Recreational scuba diving
- 300m+ / 30 ATM: Professional diving, saturation diving
Understanding ATM, Bar, and Meters
You'll see water resistance expressed as meters, ATM (atmospheres), or bar. They're essentially interchangeable: 1 ATM ≈ 1 bar ≈ 10 meters. So 5 ATM = 5 bar = 50 meters.
Some watches use feet instead of meters. 100 feet equals roughly 30 meters—barely splash resistant. Don't be fooled by impressive-sounding feet ratings.
The 30m/3 ATM Watch
This is the minimum rating for any watch labeled "water resistant." It's designed to handle accidental splashes, rain, and brief hand washing. That's it. Don't shower with it, don't swim with it, and definitely don't submerge it.
Many dress watches and fashion watches carry this rating. It's essentially a "don't panic if it gets wet" rating, not a "take it in the water" rating.
The 50m/5 ATM Watch
Slightly more robust, but still limited. You can probably swim briefly in a pool, but this rating wasn't designed for water activities. Avoid diving into the water, water sports, or hot tubs. Showering is risky due to water pressure and temperature changes.
The 100m/10 ATM Watch
Now we're getting somewhere. A 100m rating is suitable for recreational swimming, snorkeling, and general water activities. This is the minimum rating most watch enthusiasts recommend for any water exposure beyond hand washing.
However, 100m is still not a true dive watch rating. Don't use it for scuba diving or high-impact water sports.
The 200m/20 ATM Watch and Beyond
At 200 meters, a watch is suitable for recreational scuba diving and serious water sports. Most purpose-built dive watches start at this rating. They feature screw-down crowns, reinforced cases, and thick crystals.
300m+ watches are designed for professional diving, including saturation diving. Brands like Rolex, Omega, and Blancpain make dive watches rated to 300m or more for serious underwater use.
Water Resistance Degrades Over Time
Here's what many people don't realize: water resistance isn't permanent. Gaskets age and harden, crowns develop wear, and crystal seals can loosen. A watch rated to 100m when new might only be good for 30m after years without service.
Have water resistance tested annually if you regularly expose your watch to water, or every 2-3 years for occasional exposure. Many watchmakers and dealers offer pressure testing for a modest fee.
The Crown Factor
A watch is only as water-resistant as its weakest point. Screw-down crowns provide much better protection than push-pull crowns. If your crown isn't screwed down tightly, your 200m dive watch might as well be a 30m dress watch.
Always ensure crowns are fully screwed in before water exposure. Never operate the crown (to change time or date) while the watch is wet or submerged.