Water Resistance Ratings Explained

A depth number printed on a dial is a static pressure test, not a promise about how deep you can swim. Two separate ISO standards govern the claim: ISO 22810 for ordinary watches and ISO 6425 for divers' watches. They test to very different things.

Two standards, two scopes

Water resistance in modern watches traces to two documents from ISO/TC 114 (Horology). ISO 22810:2010 is the general standard; per ISO it establishes the requirements and specifies the test methods used to verify water resistance and indicates the marking a manufacturer is authorized to apply. It was the first edition of ISO 22810, cancelling and replacing ISO 2281:1990 as a technical revision. It applies to ordinary, non-dive watches.

ISO 6425 is the separate divers'-watch standard. The current edition is ISO 6425:2018, published October 2018 at 12 pages, replacing ISO 6425:1996 from July 1996. It applies to divers' watches designed to withstand diving in water at depths of at least 100 m and equipped with a secured measuring system to indicate the diving time, visible in darkness. That 100 m figure is the floor a watch must clear before it can be called a diver's watch at all.

What a rating on an ordinary watch means

The number is not a swimming depth. Using ISO's own explainer, if a watch is said to be water resistant to 30 meters (100 feet), it means the case should not leak during all aquatic activities down to a depth of 30 meters. It is a laboratory pressure equivalent, measured on a still watch in a test rig, not a rating for diving, jumping, or moving a crown underwater.

ISO 22810 gives manufacturers unusual latitude. For the first time in the standard, manufacturers are free to choose the tests and sampling plan, on condition that the end product meets the requirements of ISO 22810. There is no single mandated protocol they must follow.

The standard's sample text does describe an overpressure procedure: pressurise within 1 min to a minimum overpressure of 2 bar, hold at that pressure for 10 min, then reduce it within 1 min, with higher overpressure values specified by the manufacturer and marked on the watch. A separate shallow-immersion step submerges the watch to a depth of 10 cm ± 2 cm for a minimum time. Note that the widely repeated "50 µg/min air-flow" leak criterion belongs to the predecessor ISO 2281 and is not confirmed in the ISO 22810:2010 text, so treat it with caution.

The key difference: ISO 6425 tests above the printed number

The single most important thing to understand about diver ratings is that ISO 6425 tests the watch harder than its marking. The overpressure procedure applies an overpressure of 125% of the rated pressure within 1 minute, maintained for 2 hours; the overpressure is then reduced to 0.3 bar within 1 minute and held for 1 hour, after which the watch is removed and dried with no evidence of water intrusion or condensation allowed.

In plain terms: a watch marked 200 m must withstand a test pressure equivalent to 250 m of water without leaking or showing condensation. A 22810 watch marked 200 m is not required to clear that margin. This is why two watches with identical dial numbers can sit on very different footings.

The other ISO 6425 tests, with numbers

A diver's watch has to pass more than a pressure check. The verified figures from sources quoting the standard include:

• Reliability under water: immersed to a depth of 30 cm ± 2 cm for 50 hours at 18 °C to 25 °C.
• Thermal shock: immersion in 30 cm ± 2 cm of water at 40 °C, then 5 °C, then 40 °C again, with each transition taking no more than 1 min.
• Legibility: the time must be readable in total darkness at a distance of 25 centimeters.
• Attachment strength: a force of 200 N (45 lbf) is applied to each springbar or attaching point in opposite directions, bracelet closed, with no damage.
• Magnetic resistance: per ISO 764, the watch is exposed to a magnetic field of 4,800 amperes per metre.
• Bezel: unidirectional, tested to a minimum of 100 m static at 125 percent overpressure, turning only counter-clockwise, with a minimum of 60 graduations, holding position under accidental contact.

Watches that conform are marked DIVER'S WATCH L M (or DIVER'S L M) specifically to separate real diving watches from look-alikes not suitable for actual scuba diving. If you are cross-checking a case against its bezel or clasp, our journal and the bracelet and clasp reference elsewhere on the site cover the hardware side.

One figure we will not state yet

The salt-water corrosion test is where sources disagree, almost certainly because of the 1996-to-2018 revision. Older and secondary sources describe a 30 g/l NaCl solution held for 24 hours at 18 °C to 25 °C, salinity comparable to normal seawater. A more recent source describes the 2018 corrosion test as based on ISO 9227:2017, exposing the watch with its bracelet installed to a 5% sodium-chloride solution for 48 hours. Those are not the same test (roughly 3% for 24 h versus 5% for 48 h). Because the authoritative ISO 6425:2018 text is paywalled, we are not publishing a single number here until it can be checked word-for-word.

What the rating does not tell you

ISO does not certify watches. Brands may self-test to either standard without third-party certification, so a marking asserts conformity to a method rather than an independent audit. A rating is also a snapshot of a new, sealed case: gaskets age, crowns get bumped, and crystals loosen, which is why servicing and pressure re-testing matter over a watch's life. For how those intervals actually play out, see the blog, and about this reference desk explains how we source and flag figures like the ones above.

This page is informational. Figures and rules cited are accurate as of the review date above and can change.

Last reviewed July 30, 2026.