Water resistance, honestly explained: what the number on your case back actually promises.
Junghans rates the Max Bill Automatic to 5 bar, a figure that on a dive watch would translate to roughly 50 meters, plenty of margin for a swimming pool. Junghans’s own resistance table says the opposite: washing up, rain, splashes and even a bath are fine, a shower or the pool are not, per our Atlas entry on the watch. The same number, two contradictory instructions, and both are correct. A water resistance rating is not a promise about your swim. It is the result of one lab procedure, run once, on a watch fresh from the factory, and reading it correctly is worth more than memorizing which figure looks most impressive on a dial.
What the number on the case back actually measures
Most watches carry that number under ISO 22810, the international standard for ordinary water-resistant watches, the ones not marketed as dive watches. ISO 22810 lays out two tests. The static overpressure test submerges the watch in a pressure vessel and brings it to the rated pressure within one minute, holding it there for ten minutes, or to a minimum of 2 bar if no rating is specified, per Horological Meandering’s rundown of the standard’s test text. The condensation test heats the watch to 40 to 45 degrees Celsius, then drops room-temperature water, 18 to 25 degrees, onto the crystal. After about a minute the crystal gets wiped dry, and any watch with condensation on the inside of the glass fails outright, per that same source.
Both tests happen once, on samples pulled from a production run, before the watch ever reaches a store shelf. Wikipedia’s entry on the water resistance mark states plainly that a test pressure expressed as a depth does not mean the watch was designed for repeated, long-term use at that depth, per Wikipedia’s entry on the water resistant mark. A rating is a snapshot of one watch on one day, not a warranty that travels with the watch for its whole life. That gap is also why no watch is legally sold as “waterproof” anymore. The US Federal Trade Commission adopted its Guides for the Watch Industry in 1968 to police exactly this kind of claim, and by 1997 the FTC was proposing to retire those guides on the reasoning that international standards already did the job, per the FTC’s own 1997 press release on the review. “Water resistant” is the honest term precisely because “proof” promises something a gasket cannot deliver forever.
Bar, ATM, meters: the same claim in three units
Whichever unit a brand prints, the underlying claim converts cleanly: 1 bar of pressure equals roughly 10 meters of water depth, and 1 atmosphere, ATM, equals about the same, per Wikipedia’s entry on the water resistant mark. That is why the common ratings line up in neat steps: 3 ATM or 30 meters, 5 ATM or 50 meters, 10 ATM or 100 meters, 20 ATM or 200 meters. Bar, ATM and meters are not three different tests. They are one lab result written in three units, and a brand choosing one over another is a labeling decision, not a technical one.
The number alone still tells you nothing about what the watch is for. The Cartier Tank Must carries 3 bar, about 30 meters, a rating that covers rain and a splashed sink and nothing more, and nobody buys a Tank to swim in it. The Omega Speedmaster Professional, the watch that went to the moon, carries only 50 meters, less than plenty of $150 field watches, because a chronograph built to survive a spacesuit was never engineered around a dive computer’s problem. A high number does not mean a more serious watch. It means a different job.
ISO 6425: the only rating that means dive watch
A watch that actually intends to go diving carries a different standard entirely: ISO 6425. It sets a floor of 100 meters minimum and then tests far harder than ISO 22810 does. The overpressure test runs at 125 percent of the rated pressure, held for two full hours, a margin built in for seawater’s extra density and for gaskets that have already started to age, per Aquastar’s explainer on the standard and Horological Meandering’s comparison of the two ISO tests. A 200 meter dive watch has to survive a lab pressure equal to 250 meters before it earns the rating.
That is one test among many. ISO 6425 also demands 50 hours submerged at a shallow depth with every function still working, per Horological Meandering’s comparison of the two ISO tests, plus a crown sealed to at least 25 percent of the rated pressure, resistance to a 4,800 A/m magnetic field while holding accuracy within 30 seconds a day under ISO 764, legibility from 25 centimeters in total darkness after 180 minutes of light exposure under ISO 17514, salt spray corrosion resistance, shock resistance to hammer strikes and a one-meter drop onto hardwood, and a strap or bracelet strong enough not to fail under load, per The Slender Wrist’s rundown of the full test regime and Aquastar’s explainer. The bezel has to turn only counterclockwise, in five-minute marks, a rule I cover in lesson 2.2. Pass all of it and the watch is allowed to print DIVER’S WATCH on the dial, a label ISO 22810 watches never earn, per Horological Meandering’s comparison.
The Citizen Promaster NY0086 clears the whole regime for about $350. The Rolex Submariner 124060 clears it at 300 meters, three times the ISO floor. The Casio G-Shock GW-M5610U is rated to the same 200 meters as the Citizen and will survive worse everyday abuse than either, but it carries no rotating bezel and no diver’s certification, because Casio never built or marketed it to clear ISO 6425. Two watches sharing one number, one of them a certified dive instrument and one of them a very tough digital watch. The number on the case back never told you which.
Why the rating shrinks as the watch ages
Water resistance is not a fixed property of a case. It comes from three things working together: the case construction, how tightly the crystal seats, and the condition of the gaskets sealing the crown, the caseback and any pushers, per SJX’s guide to water resistance. Gaskets are usually nitrile rubber or silicone, and both wear and grow brittle with age, so the rating a watch left the factory with is not the rating it carries five years later, per that same guide. The factory test also happens at manufacture, not at the point of sale, so a watch sitting in a case for a year before you buy it has already had time to begin aging before its own clock even starts.
Crown design matters too. A screw-down crown locks against a gasketed tube, and Rolex’s Twinlock system backs that up with a second gasket inside the crown itself, so one failed seal still leaves a working one, according to that same guide. A plain push-pull crown relies on a single gasket doing the whole job. Either way, the seal only holds while the crown is actually closed: leave it unscrewed, or press a chronograph pusher underwater, and the seal built for a static lab test is doing nothing, per SJX’s guide. One popular worry turns out backwards: an ordinary hot shower will not hurt a rubber gasket, since nitrile stays stable to roughly 100 degrees Celsius and silicone tolerates even more, well above a shower’s usual temperature, per that same source. What actually damages a gasket is years of soap, sweat and swings in temperature working on it slowly, per M Diamond’s guide to water resistance service, which also notes that watchmakers like Hamilton recommend a yearly pressure test for anyone who swims in a watch regularly, and that a routine gasket check runs under $100 against a water-damage repair that can run $300 to well over $1,000 on a high-end piece.
Reading a rating before you swim, shower, or dive
I would treat any number under 100 meters as a promise about weather, not water: rain, hand washing, a caught splash, nothing that involves submerging the watch on purpose. Above 100 meters and marked simply in meters, I would still keep it to swimming and showering rather than diving, since a bare meters figure with no DIVER’S marking has cleared none of the extra tests that matter underwater. The only number I would trust for actual diving is one paired with ISO 6425 certification and the bezel, crown seal and legibility testing that comes with it. And on any watch I wear regularly near water, I would get the gasket checked and pressure tested about once a year, since the number printed on the case back is only ever as current as the last time someone actually tested it.
Questions worth asking before you buy
What does a water resistance rating on a watch actually mean? It is the result of a one-time lab pressure test on a sample from a production run, usually under ISO 22810, not a guarantee of how the watch performs years into ownership or during an actual swim.
Is 50 meters water resistant enough to shower with? Often, but not always. Junghans rates its Max Bill Automatic to 5 bar and still tells owners to skip the shower and the pool, so check the manufacturer’s own guidance rather than assuming from the number alone.
What is the real difference between ISO 22810 and ISO 6425? ISO 22810 covers ordinary water-resistant watches tested once at their rated pressure. ISO 6425 covers certified dive watches, tested to 125 percent of rated pressure for two hours, plus magnetic, shock, salt-spray, legibility and bezel requirements ISO 22810 never touches.
Can I go swimming with a watch rated 30 meters? Not by design intent. 30 meters, 3 bar, is the tier meant for splashes and rain, and most manufacturer guidance treats it as too low for swimming or showering.
Does water resistance actually get worse over time? Yes. Gaskets dry out and lose elasticity with age and chemical exposure, so a watch tested to 100 meters at the factory may not hold that rating years later without a fresh pressure test.