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

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

The number on the dial or case back is a test rating, not a depth promise. The table translates each level into situations you recognize. It is intentionally conservative: it shows what the level can handle according to the manufacturers' own instructions, not what might happen to work. If you want to go straight to watches built for water, you can find the 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, 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 of a ten-meter water column. A watch marked 10 ATM has therefore passed a pressure test equivalent to one hundred meters of water under controlled conditions.

The test is done on a new watch in still water at a constant temperature. The watch does not move; there is no soap, no salt water, and no temperature fluctuation. That 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 during a swim stroke, jump in from a pier, or hold your wrist under a shower stream, the momentary pressure against the seals is higher than the pressure at the depth the number indicates. This is the most common reason why a watch that "should" handle water still gets condensation under the glass.

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

Temperature also plays a part. Hot water makes metal and rubber expand at different rates, and in a sauna or under a hot shower, a seal may allow moisture through that it would otherwise have stopped. Material choice affects how the watch fares over time – we cover the differences in the guide on watch materials.

Can you shower with a 5 ATM watch?

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

The safe advice: read the instructions for your specific model. If they are gone, assume splashes and rain only. Moreover, soap, shampoo, and hot water are a greater strain than the pressure itself, as they break down the rubber in the seals faster than clean cold water.

What marking is required for diving?

It is not the number that decides, but whether the watch is tested as a diver's watch. Regarding the G-Shock, Casio writes that the 200-meter rating of standard models fits many water activities, but that diving requires a watch specifically constructed and tested for the purpose – and that it is the Frogman series that carries that dive standard. Similarly, Seiko lists scuba diving only on the Diver's Watch line, not at 20 bar.

In practice, this means you should look for the word DIVER'S on the dial or case back, not just a high number. A proper diver's watch also has a screw-down crown, a unidirectional rotating bezel, and clear luminous features. If you want to see what that 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 for all time, as seals age and parts can be deformed by impacts. A five-year-old watch that has never been opened does not necessarily hold up to what is written on the case back.

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

The routine after swimming is at least as important as the number. We have gathered the steps in our guide on cleaning your watch.

What do you do if moisture has entered under the glass?

Fogging on the inside of the glass means that the seal has allowed moisture through, and it should be taken seriously even if it disappears on its own. Water that remains attacks the movement and can cause rust damage that becomes more expensive than the watch itself to repair.

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

Can leather straps handle water?

No, and that is a point often missed. The number 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 separate.

If the watch is to accompany you into the water, steel, titanium, resin, or rubber is a better choice. If you want to maintain the leather feel for everyday use, it works perfectly to have an extra strap to swap to – the range is available among our men's watches with leather straps. The drive, however, does not matter for the tightness; we cover the differences between quartz, automatic, and solar in the guide on different movements.

How to choose the right level

Base it on how the watch will actually be used, not the highest possible number. For the office and everyday use, 3–5 ATM is sufficient. If the watch will accompany you to the pool or beach, 10 ATM is a safer level, and for snorkeling and water sports, 20 ATM is reasonable – you can see how two sports series handle it in our comparison of Citizen Promaster and Bulova Marine Star. Choose one level above your normal situation so that you have a margin as the seals age.

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

Is 3 ATM waterproof?

3 ATM means the watch handles rain and splashes, such as 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 essentially to the same pressure, and manufacturers use the units interchangeably. 5 ATM, 5 bar, and 50 m stand for the same test rating.

Is 10 ATM enough for snorkeling?

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

Can I wear the watch in the sauna?

No. Saunas, hot tubs, and hot showers should be avoided even on watches with high numbers. Rapid temperature changes cause materials to expand at different rates and can cause a seal to allow moisture through.

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 whether the watch can be used for diving is the dive marking, not the higher number.

How often should the seals be checked?

Water resistance is not permanent, as seals 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.