Water resistance ratings, decoded

A watch marked "water resistant 30 m" cannot be taken 30 meters underwater. That sentence surprises almost everybody, and it is the single most useful thing to know about the specification. The figure is not a depth limit and never was. It is shorthand for a pressure the case survived in a laboratory, held steady, for a defined time, on a watch that had just been made.

Real water is not a laboratory. Push your arm through a swimming pool and the water in front of your wrist is briefly at a far higher pressure than the depth alone suggests. Dive in and the impact spikes higher still. That gap between a static test figure and a dynamic load is why the rating is best treated as a category rather than a number.

What the number is actually measuring

The rating comes from putting the watch in a pressure vessel, raising the pressure to the equivalent of the marked depth for a period measured in minutes at room temperature, and then checking whether water got in. The watch under test is new. Its gaskets have never been compressed for years, heated in a shower, smeared with sunscreen or dragged through beach sand.

So the marking answers a narrow question: on the day it left the factory, could this design hold that static pressure? It does not answer the question owners have, which is whether this particular watch, at its current age, will survive what you are about to do with it.

Two points follow. The rating describes the case as a system: crystal, caseback, crown, pushers and every gasket between them, so one perished seal reduces the whole assembly to its weakest point. And the rating is not permanent. Nothing about the marking survives a hard knock, a careless service or a decade of aging rubber.

The units: bar, ATM and meters

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 (1.013 bar) and very close to the pressure exerted by a 10 meter column of water. Hence the conversion brands print:

• 3 ATM = 3 bar = 30 meters
• 5 ATM = 5 bar = 50 meters
• 10 ATM = 10 bar = 100 meters
• 20 ATM = 20 bar = 200 meters

The meter figure is a pressure expressed as a depth of water, which is the sleight of hand that misleads people. The watch is not being told how deep to go. It is being told how hard it will be squeezed.

ISO 22810: the standard behind the marking

The current international standard for ordinary water-resistant watches is ISO 22810, which replaced the older ISO 2281. It sets out what a manufacturer must do before a watch may carry the words "water resistant", and how that claim may be marked.

At a high level it requires:

• Marking rules. The watch may be marked "water resistant", optionally with a pressure rating in bar or its depth equivalent. The standard is explicit that a watch conforming to it is not a divers' watch, which is a separate specification covered in our guide to dive watches and ISO 6425.
• A condensation test. The watch is warmed, a drop of cool water is placed on the crystal, and the inside of the crystal is inspected for misting. Any condensation is a failure.
• Immersion in shallow water, to catch gross leaks at handwashing depth.
• Resistance of the operating devices. The crown and any pushers take a modest sideways force while the watch is immersed, because that is how people knock a watch in water.
• Resistance to a specified temperature change, moving the watch between warmer and cooler water in a defined cycle and then checking for condensation.
• Overpressure testing, in water and in air, at the pressure the manufacturer wants to claim.

The last point is the one that matters commercially: the manufacturer chooses the rating and then demonstrates that the watch meets it. There is no external body assigning the number. ISO 22810 also does not require every individual watch to be tested; sampling is normal for ordinary water-resistant watches. That is a genuine difference from the divers' standard, where every watch must be tested.

We have described those tests in general terms deliberately. The exact durations, forces and temperatures live in the standard document, and quoting them second hand is the sort of false precision that gets repeated wrongly across the internet.

What each rating allows in practice

Why heat and chemistry are worse than depth

Gaskets are elastomers. They expand when warm and contract when cool, and a compressed seal that changes dimension is a seal that can momentarily stop sealing. A shower or a hot tub does this repeatedly.

Worse is the sequence people do without thinking: a warm watch entering cold water. The air inside a warm case has expanded; cool it suddenly and it contracts, dropping the internal pressure below ambient. The case is now, very slightly, a vacuum pump, and any imperfect seal is asked to hold pressure in the direction it was never designed for. This is the classic way to fog a crystal on an otherwise healthy watch.

Chemistry does slower damage. Soap and shampoo are surfactants: they lower surface tension, helping water past a marginal seal, and over time they degrade rubber. Sunscreen, insect repellent, perfume, solvents and chlorinated water all attack elastomers, and chlorine can pit steel. Rinse in fresh water afterwards, always.

Screw-down crowns and pushers

A pushed-in crown is sealed only by an O-ring squeezed around the crown tube. A screw-down crown threads onto that tube and clamps the seal mechanically, compressing the gasket properly and stopping the crown being pulled out by accident. It is the component most responsible for a watch being trustworthy at 20 bar and beyond.

Screw-down pushers do the same job on a chronograph, which is why a diver's chronograph is stiff to operate: unscrew, press, screw back down.

The rule that follows is absolute. An unscrewed crown makes the rating meaningless. A 300 m diver with its crown a half-turn loose is, at that moment, less water resistant than a 30 m dress watch. Check the crown before water, every time, and never operate a crown or pusher while the watch is submerged unless the manual explicitly permits it, which for pushers it almost never does.

Gaskets, pressure testing, and why both are consumables

The seals are usually O-rings in nitrile rubber (often written NBR) or silicone, sometimes a fluoroelastomer in more demanding cases, sitting in machined grooves at the caseback, crystal, crown tube and pusher tubes. They work by being compressed, and rubber held under compression takes a set: it slowly loses the springiness that made it seal. Add heat, ultraviolet light, ozone, skin oils and grit, and a gasket that was perfect at five years can be glazed and cracked at ten.

This is why a rating needs re-verifying rather than trusting. A pressure test takes a watchmaker a few minutes and costs very little. There are two kinds:

• A dry test puts the watch in a sealed chamber and applies air pressure while a sensor measures how the case flexes. A case that flexes under pressure and then fails to return has let air in. Nothing gets wet and a failing watch is not damaged further, so this is the test to ask for.
• A wet test lowers the watch into water, pressurises, and watches for bubbles escaping the case. It is cheap and visual, but it finds a leak by letting water reach the case. Most watchmakers dry test first and use the wet test only to locate a leak they already know is there.

If you swim in a watch, have it pressure tested about once a year and have the gaskets replaced whenever the case is opened, a point our servicing and care guide returns to.

What kills water resistance

In rough order of how often it happens:

1. A crown left unscrewed, or not fully screwed home.
2. Age. Gaskets harden and take a compression set whether the watch is worn or not.
3. Heat cycling. Showers, hot tubs, saunas, a dashboard in the sun.
4. Soap, shampoo, sunscreen and solvents breaking down the elastomer.
5. A caseback opened and closed carelessly, with the old gasket reused, pinched or replaced with a wrong-sized one. Battery changes are the classic offender.
6. A cracked or chipped crystal, or a crystal gasket disturbed by an impact. Different materials fail differently, as the crystals guide sets out.
7. Grit in the crown threads or under the caseback, holding a seal open by a fraction of a millimeter.
8. Impact, which can deform a case enough to break the seal or loosen a caseback that felt tight.
9. A corroded or worn case, particularly plated cases where the plating has gone through, a point covered in the case materials guide.
10. Ultrasonic cleaners and pressure washers, which drive water past seals that would hold under static pressure.