An Explainer on Waterproof – Water Resistance Watch Ratings

Have you ever wondered what the 10ATM rating on your watch means? Are you going on a dive soon and don’t know how far underwater you can take your watch? Perhaps you’re a watch enthusiast trying to figure out the best watch mod parts for diving. 

Wonder no more. We’ll discuss everything you need to know about waterproof and water resistance ratings.

What’s ATM?

In order to figure out how far underwater you can go with your watch, it’s important to be familiar with ATM. ATM stands for atmosphere, which is a measurement of atmospheric pressure. 1 ATM is equal to atmospheric pressure at sea level. 2 ATM would be double that pressure, and so on. 

When you submerge your watch in water, it begins to resist the pressure exerted by the water. The deeper the watch goes into the water, the more pressure it is resisting. This is why ATM is used. It’s needed to measure how much water pressure a watch can take before breaking. 

How is it possible to determine how deep a watch can go?

Thankfully, the math to convert ATM to actual depth is simple. For every 10 ATM water resistance (WR) your watch has, it can go 100 meters deep into the water without breaking. Probably.

Probably? Why probably?

It’s important to remember that ATM is a measurement made in static laboratory conditions. They do not account for things that may change water pressure, such as temperature, water flow, or movement in general. This means that at any given depth, your watch could be taking more or less pressure than what the depth would indicate.

Don’t worry, though. In the ISO diver’s watches standard, it states that watches must be able to resist 125% of the test pressure as a safety margin. This accounts for a dynamic pressure increase caused by movement. Other additional causes include thermal shock, difference in water density (saltwater is denser than freshwater), and wear and tear of water seals.

Does this mean a 100M watch is safe at a 100 meter-depth?

A factory standard 100M-rated watch in mint condition can be used at a depth of 100 meters without breaking. Over time, gaskets begin to get worn out, which means you’re never entirely sure of how deep you can go without testing tools. Not to mention, the fact that even with testing tools, most people don’t test their watch before going to the beach. Plus, you never know how your Seiko mod parts could be affecting your watch’s WR rating.

For these reasons, most brands have decided that the WR rating is more of a guideline rather than a strict, precise measurement of how deep the watch can go. For example, some brands say that 3 ATM/30M means it’s rain-resistant, and at this level of WR rating, you can wash your hands without damaging your watch. Brands will not say that 3 ATM/30M is equal to 30 meters of diving depth.


At the end of the day, taking care of your watch is the most important thing you can do. Remember to replace the gaskets at least once per year, and wash off salt water with clean water. Also, do not take your watch deeper than it is meant to. All this will ensure peak performance. 

Are you looking for Seiko mod parts in the UK? Contact SeikoMods today! We are the crystal specialist to go to for Seiko sapphire crystals, as well as cases, casebacks, bezels, bezel inserts, crowns, chapter rings, dials, movements, and other associated metal parts.

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