What does ATM mean on a watch? How to interpret water resistance

What do 1, 3, 5, 10, 20, and 30 ATM mean?

The number on the dial or case back is a test classification, not a promise of depth. The table translates each level into situations you recognize. It is deliberately conservative: it shows what the level can withstand according to the manufacturers' own instructions, not what might happen to work. If you want to go directly to watches built for water, you'll find a selection among our sports watches.

The same level is written differently by different manufacturers. 5 ATM, 5 bar, and 50 m mean the same thing. If it only says WATER RESISTANT without a number, you should treat the watch as splash-proof and nothing more.

What does ATM actually mean?

ATM is an abbreviation for atmosphere and is a unit of pressure. One ATM corresponds approximately to the air pressure at sea level, and each additional ATM corresponds to the pressure from a ten-meter high column of water. A watch marked 10 ATM has thus passed a pressure test corresponding to one hundred meters of water under controlled conditions.

The test is performed on a new watch in still water at a constant temperature. The watch does not move, there is no soap, no saltwater, and no temperature changes. This is why the number should be read as a margin, not as a limit you can push.

Why is ATM not a depth you can swim at?

Movement creates pressure. When you pull your arm through the water in a swimming stroke, jump in from a pier, or place your wrist under a shower stream, the instantaneous pressure on the gaskets is higher than the pressure at the depth indicated by the number. This is the most common reason why a watch that "should" be waterproof still gets condensation under the crystal.

The crown and pushers are the watch's weakest point. This is where the case must be open to be set, and the sealing relies on everything being in its bottom position. A crown that has accidentally been pulled out a notch in your pocket, or a pusher that is pressed underwater, renders the stated level ineffective. On models with a screw-down crown, it must be fully screwed in, and on chronographs, the stopwatch should never be started or reset while the watch is wet.

Temperature also plays a role. Hot water causes metal and rubber to expand at different rates, and in a sauna or under a hot shower, a gasket can let in moisture that it would otherwise have stopped. The choice of material affects how the watch fares over time – we go through the differences in the guide on watch materials.

Can you shower with a 5 ATM watch?

Manufacturers differ here, and it's worth knowing before you take a chance. Seiko states in its official table that 5 bar allows showering and swimming. Other manufacturers describe the same level as enhanced everyday resistance and draw the line at splashes. Two watches with identical numbers can therefore have different permitted uses.

The safe advice: read the instructions for your specific model. If they are missing, assume it's only for splashes and rain. Soap, shampoo, and hot water are also a greater strain than the pressure itself, as they break down the rubber in the gaskets faster than clean cold water.

What marking is required for diving?

It's not the number that determines it, but whether the watch is tested as a diver's watch. Casio writes about G-Shock that the standard models' 200 meters are suitable for many water activities, but that diving requires a watch specifically designed and tested for the purpose – and that it is the Frogman series that meets that diving standard. Similarly, Seiko lists scuba diving only under the "Diver's Watch" category, not at 20 bar.

Practically, this means you should look for the word DIVER'S on the dial or case back, not just a high number. A true diver's watch also has a screw-down crown, a unidirectional rotating bezel, and clear lume. If you want to see what this construction looks like in reality, the models are gathered under Citizen Promaster and Casio G-Shock.

What destroys water resistance over time?

Water resistance is perishable. Seiko explicitly states that a watch's water resistance is not guaranteed forever, as gaskets age and parts can deform from impacts. A five-year-old watch that has never been opened does not necessarily maintain what is stated on the case back.

• Never pull out the crown or press buttons when the watch is wet.
• If the watch has a screw-down crown, it must be fully screwed in before it comes into contact with water.
• Rinse in fresh water and wipe dry after swimming in the sea, so that salt does not dry into the gaskets.
• Do not rinse under a full-blast tap – use still water in a bowl.
• Avoid saunas, hot tubs, and hot showers even with watches rated for high ATM.

The routine after swimming is at least as important as the ATM rating. We have gathered the steps in the guide on how to clean your watch.

What to do if moisture gets under the crystal?

Condensation on the inside of the crystal means that the seal has let in moisture, and this should be taken seriously even if it disappears on its own. Water that remains attacks the movement and can cause rust damage that will be more expensive to repair than the watch itself.

Take off the watch, wipe it dry externally, and take it to a watchmaker as soon as you can. Do not open it yourself or place it on a radiator – rapid heating can move the moisture further into the movement instead of out. After moisture damage, or after a battery change, the seals need to be checked before the watch comes into contact with water again.

Are leather straps water resistant?

No, and this is a point that is often missed. The ATM rating applies to the case, not the strap. A watch can be marked 10 ATM and still have a leather strap that becomes stiff, darkens, and starts to smell after a few swims. Leather absorbs water and dries unevenly, and glued layers can come apart.

If the watch is going to be in water, steel, titanium, resin, or rubber is a better choice. If you want to maintain the leather feel for everyday wear, it works perfectly to have an extra strap to switch to – the selection is available among our men's watches with leather straps. The movement, however, does not affect water resistance; we cover the differences between quartz, automatic, and solar in the guide on different movements.

How to choose the right level

Base your choice on how the watch will actually be used, not on the highest possible number. For office and everyday wear, 3–5 ATM is sufficient. If the watch is going to be in the pool or at the beach, 10 ATM is a safer level, and for snorkeling and water sports, 20 ATM is reasonable. Choose a level above your normal use, so you have a margin as the gaskets age.

If the watch is to be used for diving, choose from models that are built and tested for it – these are gathered under diving watches. If you are looking for a smaller model with higher durability, you will find a selection among our water-resistant women's watches.

Is 3 ATM waterproof?

3 ATM means the watch can withstand rain and splashes, for example when washing hands. It is splash-proof rather than waterproof: showering, bathing, and swimming are outside this level.

What is the difference between ATM and bar on a watch?

In practice, none. 1 ATM and 1 bar correspond to roughly the same pressure, and manufacturers use the units interchangeably. 5 ATM, 5 bar, and 50 m represent the same test classification.

Is 10 ATM enough for snorkeling?

10 ATM is sufficient for swimming and snorkeling near the surface. For scuba diving, it is not enough, regardless of depth – then a watch that is tested and marked as a diver's watch is required.

Can I wear my watch in the sauna?

No. Saunas, hot tubs, and hot showers should be avoided even with watches rated for high ATM. Rapid temperature changes cause materials to expand at different rates and can cause a gasket to let in moisture.

Is 30 ATM better than 20 ATM in practice?

The difference is rarely noticeable in everyday life. Both levels can handle swimming, snorkeling, and water sports. What determines if the watch can be used for diving is the dive marking, not a higher number.

How often should seals be checked?

Water resistance is not permanent, as gaskets age. If the watch is used in water regularly, it is reasonable to have a watchmaker check the seals every few years, and always after the case has been opened.