What each water resistance rating means, and why 30 metres is not 30 metres

A watch marked 30 metres is not designed to go down to 30 metres. According to the usual table of markings, 30 m or 3 bar covers daily wear, splashes and rain, not showering or swimming. The Casio F-91W, which carries that marking, is the classic example.

Every 10 metres of marking equals 1 bar. What gets measured is static pressure in a machine, not a person moving an arm under a shower, with hot water and soap involved. There are two standards: ISO 22810 for water-resistant watches in general and ISO 6425 for dive watches. They are paid documents we have not been able to read directly; what follows comes from public summaries of both, and from what the brands say.

The practical table

The gap in the second row is no small detail: at 5 bar, one source allows gentle swimming while a maker of inexpensive watches talks about hand washing. Casio's own pages do not always agree with each other on 20 bar. When in doubt the watch's manual wins, not this article or any general table.

Why a dive watch is something else

A watch can withstand a lot of pressure and still not be a dive watch. ISO 6425 requires, according to its public summary, a minimum of 100 metres and a series of tests a city watch does not pass: 50 hours submerged at about 30 cm, alternating immersions at 40, 5 and 40 degrees, and a 125% overpressure for two hours. That margin is no whim: it covers dynamic peaks, salt water, which is 2% to 5% denser, and gasket ageing.

The rule shows in extreme watches: the Deepsea, rated to 3,900 m, is tested at 4,875 m, and the Deepsea Challenge, rated to 11,000 m, at 13,750 m. Exactly 125%.

Watches for saturation diving add another test: pressurised gas for 15 days, and they leave the factory with the mixed-gas diving inscription.

The normal and the absurd, on a single scale

Note the second figure: the Speedmaster Moonwatch, the watch that went to the Moon, is rated to 50 metres. Less than many sports watches that will never leave an office. Nor is that a flaw, because it is a chronograph, not a dive watch.

What to do with all this

• For true daily wear, 100 metres is the comfortable figure. It is what lets you swim with a watch without a second thought, according to both the usual table and Casio. This is our advice, not a standard.
• A water resistance rating expires. Gaskets age, as the very logic of the ISO 6425 margin recognises. An old watch engraved "300 m" does not guarantee 300 m today.
• Check whether it says "Diver's". A high figure does not make a watch a diving instrument; the standardised inscription does signal a specific test.

Two curiosities to finish. The Nautilus of 1976 already advertised 120 metres, more than double a current Moonwatch. And three prototypes of Omega's Ultra Deep survived a 10,928-metre dive into the Mariana Trench in 2019, although that family's rating stops at 6,000.