What watch water resistance ratings actually mean
Every depth figure on a caseback was measured in a tank of still water, on a watch that had never been worn. Here is what each rating proved, and what it never covered.
Short answer
Watch water resistance ratings are pressure tests, not depths you can visit: “100 m” means the model held the pressure of 100 m of still water in a lab. A bare “water resistant” mark is a 2 bar test — rain and hand washing only. The rating belongs to the model when new, and the seals that deliver it age.
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What the number on the caseback actually measures
Water resistance on an ordinary watch is governed by ISO 22810, which sets out how water-resistant watches are designated, tested and marked; the current edition, ISO 22810:2010, replaced the older ISO 2281. The test is static: the case is held at a set overpressure in a rig, and what is measured is whether the seal holds. Nobody takes it swimming.
So the depth on the back is a unit of pressure in disguise. One bar is roughly the pressure of ten metres of still water, which makes 10 bar, 10 ATM, 100 m and “WR 100” four ways of writing one test result — the short version, with the other caseback markings, is in our guide to the back of the case. Summaries of the standard are blunt about the translation: a test pressure expressed as a depth does not mean the watch was designed for repeated, long-term use at that depth.
Two things push a real wrist past the number. Movement is one: a swimming stroke, a jump into a pool or a shower jet aimed at the crown all raise the pressure above the still-water figure, and the test was a submersion, not a spray. Temperature is the other: hot water makes the metal expand and contract, which works the seals.
You will not see “waterproof” on a modern watch: the industry dropped the word because it claims more than any case can keep. Timex’s own guide draws the line as resisting water to a degree rather than absolutely.
What each water resistance rating lets you do
Below are the common markings, what the test behind each established, and what it does not settle. Where brands and sellers disagree, both readings are given.
| Marking | What the test proved | What it doesn’t settle |
|---|---|---|
| “Water resistant”, no figure | It held an overpressure of 2 bar — about 20 m of still water — the baseline the standard attaches to a bare mark | Anything past rain, splashes and washing your hands. Sellers treat everything up to 3 bar as splash protection |
| 50 m / 5 bar / 5 ATM | The same static test at the pressure of 50 m of still water | The most contested rating. Some brand FAQs allow a calm, shallow swim with no jumping or diving; other sellers advise against prolonged swimming at all, and several say take it off in the shower |
| 100 m / 10 bar | The same test at the pressure of 100 m of still water | Timex’s guide puts swimming and snorkelling here, and warns against pressing any button under water unless the watch is rated to 200 m |
| 200 m / 20 bar and above | The same test, deeper; these cases usually close with a screw-down crown | Timex puts recreational scuba here, but a depth figure on its own is not the diver’s-watch standard — that is a separate specification |
| “Diver’s 200 m” / ISO 6425 | That an external standard was applied, not just a pressure number | Still only the watch as it left the factory. A thirty-year-old diver with no service history is an old watch, not a 200 m one |
Markings vary by brand and era, and a manual beats a table: check your own model. Much of the published guidance comes from firms that sell watches or servicing.
Can you shower or swim with a water-resistant watch?
Swimming is the easier question. At 100 m and above, mainstream guidance says yes to swimming and snorkelling. At 50 m the sources split, so follow the cautious reading: calm water, no diving, no jumping in. Below that, no.
The shower is where opinions divide hardest, and the arguments against are not about depth at all:
- Direct spray. A showerhead can put more pressure on one spot of the case than swimming does, and the test behind the rating was a uniform squeeze, not a jet.
- Heat and steam. Vapour reaches gaps liquid water does not, and the hot-to-cold swing works the seals.
- Soap. Shampoos, gels and soap residue attack the rubber of the gaskets over time — the parts the whole rating depends on.
- It is slow. Modern cases are good enough that damage shows up months later, which is why the habit survives.
The counter-argument, from watchmakers and dive-watch owners, is that a case with sound, recently checked gaskets and a closed crown handles a shower without drama. Both camps agree on the conditions, which is the useful part: a known-good seal, a screwed-down crown, no hot water, a rinse in fresh water after chlorine or salt — and never move the crown or press the pushers while the watch is wet.
A screw-down crown that was never screwed fully home is a routine cause of water inside a case. Check it by feel before water, every time, and have a crown that will not seat properly looked at rather than lived with.
What makes a watch a diver’s watch
A dive watch is not just a watch with a big number on it. ISO 6425 — current edition 2018, reconfirmed in 2024 — covers watches made for diving at depths of at least 100 m, and asks for more than pressure: a secured system for measuring elapsed dive time that stays visible in darkness, which is the rotating bezel and luminous markers you can picture. A separate annex covers saturation diving.
So “Diver’s 200m” is a different kind of statement from “200 m”: it claims conformity with that standard. Some brands test to their own procedures and still use the word, so the marking is evidence rather than proof — and it says nothing about this watch today. On a second-hand diver the rating is one line on a longer list; the checklist before you pay covers the rest.
Does water resistance wear off?
Yes, and it is the most useful thing to know about the figure on your caseback. Resistance comes from rubber gaskets at the crown, the caseback, the crystal and any pushers. They harden with age, take damage from chlorine, salt water and soap, and are fatigued by heat-and-cold cycling. Manufacturers say so themselves, which is why water resistance is not guaranteed for the life of the watch.
How often to renew them is where published advice spreads out. Several brands recommend replacing the gaskets annually for a watch used heavily in water, ideally before the swimming season; others say check yearly and replace every two to three years, or at 12 to 18 months for a dive watch. All agree the seals go whenever the case has been opened. Most of that advice comes from firms that sell the service, so read the intervals as starting points.
What settles it for your watch is a pressure test — dry, in a vacuum chamber watching for crystal movement, or wet, pressurising the case under water and looking for bubbles. It costs a fraction of a service and is the only way to know whether a rating stamped on a caseback decades ago still describes anything. How often an automatic watch needs servicing covers where that test sits in the schedule. Misting under the crystal is not a wait-and-see: water is already inside.
If the watch was handed down rather than bought, treat the rating as unproven until it is tested: what to do first with an inherited watch starts with keeping it dry.
How to find your watch’s rating when the caseback is worn
The rating belongs to the model, not to your individual watch, so you do not have to read it off the steel:
- Get the reference. The model code or reference number pins down the exact version, including the case — see where to find a watch’s reference number.
- Look up that reference. The brand’s page or manual gives the figure the watch was marked with when new — more reliable than a half-polished engraving.
- If the numbers have gone, identify the model. The dial, bezel, hands and case shape narrow it down — how to identify a watch from a photo compares the ways.
- Then test, don’t assume. The specification says what the model was rated for; only a pressure test says what your watch does now.
Don’t know which model you have?
Loupe identifies the brand, model and reference from a photo, with a confidence rating on every result, so you can look up what your watch was built for. It also gives an estimated resale range — not an appraisal — and keeps your collection on your phone. Free trial, cancel in Google Play.
