Watch water resistance explained: 200 metres does not mean 200 metres, and the makers say so themselves

The short version

Three facts clear up almost every question people have about this.

One: the number is a pressure, converted into a depth for marketing. 10 bar is 10 atmospheres, which is the pressure at roughly 100 metres of still water. The watch was held at that pressure, motionless, at a set temperature. You moving your arm through water adds pressure the test never applied.

Two: the makers publish what the rating permits, and they are more conservative than you expect. Casio states that 20 bar lets you water ski, surf and snorkel but is not suitable for extended scuba diving. That is 200 m on the caseback and a no to scuba, from Casio.

Three: a dive certification is a different and stricter claim than a pressure rating. Citizen describes the Promaster BN0151 as an ISO-certified dive watch at 200 m; Amazfit publishes 10 ATM to ISO 22810 tested by SGS plus a separate freediving certification to 45 m under EN13319 and ISO 6425. A certification names the standard and often the lab. A bare pressure number names neither.

What each rating allows

Reading across Casio's own guidance, with the markings you will actually see on a caseback:

The practical takeaway is simple and slightly counterintuitive: the gap that matters most is between 5 bar and 10 bar, not between 10 bar and 20. Going from 5 to 10 is the difference between a watch you hesitate about and a watch you forget about. Going from 10 to 20 mostly buys margin and peace of mind, unless you dive, in which case you need a certification rather than a bigger number.

Why the number is not a depth

Four things the static test does not include, each of which raises the real pressure on the seals:

• Movement. Swinging an arm through water, a dive entry, a hose stream: all produce transient pressures well above the static figure at the same depth.
• Temperature. Gaskets stiffen in cold and soften in heat, and a hot tub then cold water is the classic way to kill a seal that would survive either alone.
• Age. The rating describes a new watch. Rubber gaskets compress, dry and take on a permanent set. A twenty-year-old 200 m watch with original gaskets is not a 200 m watch.
• Operation. Pressing a button or pulling a crown underwater is how water gets past a seal that would otherwise have held. This is the actual, boring cause of most flooded watches.

The standards behind the number

Three names come up on the watches we rank, and they mean different things.

ISO 22810 is the standard for water-resistant watches in general, the ordinary 5, 10 and 20 bar kind. Amazfit cites it for the T-Rex 3's 10 ATM rating and names SGS as the third-party test house, which is unusually transparent for this market and worth crediting.

ISO 6425 is the diver's watch standard, with requirements beyond pressure: it covers things like legibility in darkness, resistance to salt water and magnetic fields, and the condensation and thermal shock tests. A watch permitted to be marked DIVER'S has met this, and a 200 m watch that is not so marked has not.

EN13319 is a European standard for diving accessories, including depth measurement, which is why it turns up alongside dive ratings on watches that measure depth: Apple cites it for the Ultra 3 at 40 m and Amazfit cites it for the T-Rex 3's freediving certification.

Garmin takes a different route on the fenix 8: rather than a bare pressure number, the release describes a 40-metre dive-rated case with leakproof metal buttons. That is a smaller number than 200 m and a bigger claim, which is exactly the point of this section.

What the watches on this site are rated for

Two patterns are worth noticing. Every Casio on this site, including the cheapest, is 200 m, which is a large part of why G-Shocks have the reputation they do. And every running watch is 5 ATM, because a running watch is designed to be rained on and swum in, not worn into surf. Choosing between those two worlds is the subject of Instinct vs Forerunner.

The picks

Keeping the rating you paid for

Five rules, all of which come straight out of how the test differs from real life.

1. Never press a button or pull a crown underwater. This is the single most common way a sealed watch floods, and it is entirely avoidable.

2. Rinse in fresh water after salt. Salt crystals dry in the button gaps and abrade the seals. Ten seconds under a tap is the highest-value maintenance habit there is.

3. Avoid hot water and sudden temperature swings. Hot tubs, saunas and hot showers are harder on gaskets than swimming is, because heat and thermal shock both work against the seal.

4. Treat gaskets as a wear part. If a watch is going to be used in water for a decade, a pressure test and reseal at the same time as a battery or cell service is cheap insurance. The service question for solar Casios is covered in G-Shock solar battery replacement.

5. Do not trust an unserviced vintage rating. A 200 m watch from 2006 with original gaskets is a splash-resistant watch until somebody proves otherwise.

Two common versions of this question have their own pages. For paddling, where the watch is submerged and pressed hundreds of times an hour, see best watch for kayaking. For the single most asked version of all, see can you swim with a G-Shock.