Water resistance

Water resistance is a watch's ability to keep water out, expressed as a pressure in meters, feet or bar. The number is the result of a static pressure test in a tank, not a depth you may safely go to. A watch marked 30 meters has been shown to withstand a pressure equivalent to 3 bar under still conditions, and it is not a swimming watch.

Units and markings

Three units get used interchangeably and they are near enough the same thing. One bar is 100 kilopascals, which is very close to one standard atmosphere and very close to the pressure exerted by a ten meter column of water, so 3 ATM, 3 bar and 30 meters all describe the same test. Feet appear on US-market dials, which is why a 100 meter watch is often marked 330 ft.

The wording matters too. "Water resistant" with no figure is the weakest claim on a dial, a rating printed on the caseback is just as valid as one on the dial, and a watch meeting the divers' standard may be marked DIVER'S WATCH with its depth, which is a claim about equipment rather than sealing.

What the ratings are based on

ISO 22810 is the general standard for water-resistant watches, setting out the tests a watch must pass to carry the marking. ISO 6425 is the far stricter standard for divers' watches: among other requirements, every individual watch is tested rather than a sample, testing is done at 125 percent of the rated pressure, and the watch must also survive condensation, thermal shock and immersion in salt water, as well as meeting requirements for legibility in the dark and for a timing bezel.

A rough reading of the common markings:

• 30 meters, 3 bar: rain, splashes and hand-washing.
• 50 meters, 5 bar: the same plus brief incidental immersion. Most makers still do not call this a swimming rating.
• 100 meters, 10 bar: swimming and snorkeling.
• 200 meters and above, with a divers' marking: scuba diving.

Why real life is harsher than the number

The test is static, and swimming is not. Diving into a pool, or simply sweeping a hand through water at speed, briefly raises local pressure well above the static figure. Hot showers are worse than they look: heat expands the air inside the case and ages the gaskets, and the cooling afterwards can draw moisture in past a seal that was already tired. Soap, shampoo and sun cream degrade rubber.

Getting it tested

Pressure testing is quick bench work and does not involve opening the watch. A dry test seals the watch in a chamber, raises the air pressure and measures how far the case deflects, which reveals a leak without wetting anything. A wet test lowers the watch into water under pressure and watches for bubbles. Both are cheap: as indicative US figures before sales tax, a test alone is about $25 to $60, or $40 to $80 with a battery change, per our service costs.

The sensible interval is annually if you swim in the watch, and always after the case has been opened, which our answer on how often to pressure test goes into. Gaskets are the consumable in the system: nitrile or similar rubber O-rings at the caseback, crystal and crown tube, lightly greased, replaced at service rather than inspected and reused.

Common misconceptions

A rating describes the watch as tested, not the watch as it is now. Gaskets are consumables, they harden and compress with age, and a case that has been opened has by definition had its seals disturbed. If you swim with a watch, have it pressure tested every year or two and have the gaskets replaced at service.

A screw-down crown left unscrewed voids the rating entirely, and no rating at all applies while a crown or pusher is pulled out. Chronograph pushers are the same trap: unless the maker says otherwise, they are not to be pressed underwater.

The word waterproof disappeared from watch marketing decades ago, under pressure from regulators including in the US, because no case is proof against water indefinitely. Where you still see it, it is a period marking or loose copywriting. A helium escape valve is another commonly misread feature: it exists for saturation divers decompressing in a chamber and does nothing for swimming, so its presence tells you about the watch's styling more than about its sealing.

For the everyday cases, see our answers on swimming with a 50 m watch, showering with a watch and whether 100 m is enough, and the full guide for the case as a system.