Water resistant to 3, 5, 10 or 20 ATM: can you really swim with it?
Turn your watch over. There is a good chance you will read "WATER RESISTANT 3 ATM", or "30 M". Thirty metres: enough to go far deeper than the bottom of any swimming pool, you might think. It is one of the most widespread misunderstandings in watchmaking. A watch marked 30 metres should not go under water. Not even in the shower.
The rule fits in four lines. At 3 ATM, the watch copes with rain and hand-washing, nothing more. At 5 ATM, a shower and a gentle dip. At 10 ATM, you really can swim, snorkel or sail. At 20 ATM, vigorous water sports and freediving. And for scuba diving, you need a certified diver's watch, which falls under a separate standard. Here is the full table, the reason for the gap between the figure and reality, and the habits that keep water out.
3, 5, 10, 20 ATM: what you can really do
| Marking | What is allowed | What is not |
|---|---|---|
| No marking | Nothing: the watch must be kept away from water | Avoid even drops and sweat |
| 3 ATM, 3 bar or 30 m | Rain, splashes, hand-washing | Shower, bath, swimming, diving |
| 5 ATM, 5 bar or 50 m | Shower, bathing, shallow-water swimming | Diving |
| 10 ATM, 10 bar or 100 m | Swimming, snorkelling, sailing, water sports | Scuba diving |
| 20 ATM, 20 bar or 200 m | Vigorous water sports, freediving | Scuba diving, unless marked "Diver's" |
| Diver's 200 m or 300 m | Scuba diving | Saturation diving, reserved for models built for it |
This table follows the usage accepted by manufacturers. Seiko, for example, states in its instruction manuals that a watch simply marked "WATER RESISTANT" withstands accidental contact with water in everyday life and should not be used for swimming, that a "5 BAR" watch is suitable for sports such as swimming, and that a "10 (20) BAR" watch is suitable for diving without air cylinders.
Why "30 metres" does not mean 30 metres
The figure engraved on the back is not a depth you can use. It is a test pressure. It gives the static pressure that a sample of newly made watches was exposed to in the laboratory, in a leak test. Every word matters.
- Static: the watch is still, in still water. Nobody swims like that.
- A sample: for an ordinary watch, pieces are not all tested one by one. Checking is done by sampling.
- Newly made: the test takes place once, as the watch leaves the factory. It says nothing about the watch five years later, when its gaskets have aged.
The tests laid down in the earlier standard give an idea of what is asked: an hour's immersion under ten centimetres of water, ten minutes at the stated pressure, then a drop of water placed on the crystal to check that no condensation forms inside. It is rigorous, but a long way from a dive followed by twenty lengths.
The word the standard forbids
These rules come from the International Organization for Standardization. A first standard, ISO 2281, dates from 1990 and only covered watches for everyday use. It was replaced by ISO 22810, published in 2010. One telling detail: the standard forbids the use of the word "waterproof" for a watch. A watch is never waterproof. It is water resistant, up to a point and for a time.
ATM, bar, metres: three units for one idea
All three mean the same thing. One atmosphere and one bar are two practically equal units of pressure, and ten metres of water exert about one extra bar. Hence the equivalences found on every case back: 3 ATM for 30 metres, 5 for 50, 10 for 100, 20 for 200. Whichever unit your watch uses makes no difference to what it can take.
A real diver's watch meets a different standard
The word "Diver's" followed by a depth is not decoration. It refers to ISO 6425, which is far more demanding. The watch must withstand at least 100 metres and, above all, every single piece is tested individually, at 125% of the stated pressure, held for two hours. A "Diver's 200 m" has therefore really been through the equivalent of 250 metres.
The standard does not stop at water. It requires a unidirectional rotating bezel marked over 60 minutes, to keep track of dive time with no risk of lengthening it by mistake, legibility at 25 centimetres in the dark, and resistance to shocks, magnetic fields and 24 hours of salt water. That is the whole difference between a watch with the look of a diver and a watch certified for diving.
What wears water resistance down, day after day
Seiko says it plainly: a watch's water resistance is not guaranteed for life. It depends on the ageing of the gaskets and on the deformation a knock can cause. In other words, a watch can lose its water resistance without its owner doing anything out of the ordinary: it only has to grow older without anyone checking its gaskets.
A few habits are enough to put the odds in your favour:
- Check the crown. It must be pushed fully home before any contact with water. A watch washed with the crown pulled out lets water in, whatever is engraved on the back.
- Avoid the running tap. Seiko advises against washing the watch directly under running water.
- Rinse after the sea. A dip in fresh water, then careful drying.
- Have the gaskets checked. For diver's watches, most manufacturers recommend a pressure test every year or every two to three years, with the seals replaced. For a town watch, the periodic overhaul of the movement is the right moment to ask for it. We cover this in our guide to the movements in our watches.
The warning sign is unmistakable: condensation under the crystal. It means moisture has got in. Do not wait to have it checked: moisture is the enemy of a movement.
What about SmodParis watches?
We would rather be clear on this point, because it avoids disappointment. Our GMT Mod is rated at 3 ATM: it withstands rain and splashes, but it must not go under water. Take it off before the shower, the pool and the beach. That is a common rating for a town watch.
At the other end of the scale, the product page of our CasiOak, built around the Casio GA-2100 module, states 20 bar, or 200 metres. For each model, the product page is the reference. And when a page gives no figure, apply the most cautious rule: treat the watch as a 3 ATM. When in doubt, caution remains the best rule. Our watches are hand-assembled in Paris and covered by a 2-year warranty.
To get to know yours better, read also automatic or quartz, which to choose and why an automatic watch stops overnight.
Frequently asked questions
Can you shower with a 3 ATM watch?
No. A 3 ATM watch, or 30 metres, copes with rain and the splashes of hand-washing. It is not made for the shower, the bath or swimming.
Can you swim with a 5 ATM watch?
Yes, for a gentle dip in shallow water, provided the crown is pushed fully home and the watch is in good condition. To swim regularly or take up a water sport, aim for 10 ATM.
What does 10 ATM water resistant mean?
The watch was designed for a test pressure of 10 atmospheres, the equivalent of 100 metres of water in the laboratory. In practice, it is suitable for swimming, snorkelling and sailing, but not for scuba diving.
What is the difference between ATM and bar?
None for the wearer. They are two almost equal units of pressure, and manufacturers use one or the other. 5 ATM, 5 bar and 50 metres mean the same level of water resistance.
Does a watch's water resistance last a lifetime?
No. Gaskets age and a knock can deform a part. Water resistance has to be checked and maintained: a pressure test and new gaskets at each overhaul bring it back up to standard.
Sources
- Wikipedia, "Water Resistant mark" (ISO 2281, ISO 22810 and ISO 6425 standards, tests, table of uses): en.wikipedia.org/wiki/Water_Resistant_mark
- Seiko, instruction manual for calibres 4R35 and 4R36, water resistant performance (French edition): seikowatches.com
- Seiko, frequently asked questions, water resistance (ageing of gaskets, crown, running water, rinsing after the sea): seikowatches.com
- Photo: Pexels, Pixabay Licence, via Pixabay (pixabay.com). Other photo: SmodParis workshop.
Seiko and Casio are the makers of the movements and modules mentioned. The water resistance ratings of our watches are those stated on their product pages. SmodParis is an independent workshop, not affiliated with these brands.