01 · The problemWhy the numbers lie.
The international standard ISO 22810 defines water-resistance tests for watches under static conditions: the watch is submerged in a pressurized tank of still water, without sudden movement. A "30 m" watch passes this test. But in real life, water is never still.
Swimming, diving, accidentally knocking your wrist against water — every dynamic action generates pressure spikes far greater than the nominal depth. A simple pool-edge dive can generate instantaneous peaks of 5-10 ATM. A sharp wrist movement while swimming can reach 2-3 ATM peaks. Moving water is aggressive.
The practical upshot is that the stated rating should be halved — at minimum. A 3 ATM watch is not a swimming watch: it resists splashes and rain. 10 ATM is the minimum for carefree swimming. See the table.
02 · The ATM tableWhat you can actually do.
ATM (atmosphere) and BAR are practically synonymous: 1 ATM ≈ 1 BAR ≈ 10 metres of water under static pressure. But remember: the rating is static, real use is dynamic. Here's what practice says:
Splashes only
Rain, hand-washing, accidental splashes. No shower, no pool, no bath. Typical of dress watches and elegant timepieces.
3 ATM · 30 mSwimming (careful)
Surface swimming in a pool or calm water — no diving, no snorkelling. Shower OK (with crown screwed down). No water sports.
5 ATM · 50 mWater sports
Swimming, snorkelling, water skiing, sailing. The minimum for everyday water use. Not suitable for scuba diving.
10 ATM · 100 mScuba diving
Sport diving to 40 m (ISO 6425 Diver standard). 200 m+ for professional and saturation diving.
20 ATM · 200 m+Note: the BAR markings on modern watches (3 BAR, 5 BAR, 10 BAR, 20 BAR) correspond exactly to ATM. The double marking 30 m / 3 ATM you often see is the same thing expressed in two different units.
03 · The crownThe weak point of every watch.
The crown (the side button used to wind and set the time) is the most vulnerable water-entry point on any watch. The crown tube — the channel that runs through the case and lets the crown move — is sealed by gaskets, but remains structurally the most critical point.
This is why ISO 6425-certified dive watches must have a screw-down crown: it threads into the case, creating a second mechanical seal like a jar lid. Never operate the crown underwater, even on high-rated watches: setting the date in the pool or at sea is one of the most common and expensive mistakes. Always check the crown is fully screwed down before entering water.
The water-resistance rating is certified with the crown fully closed. If your watch has a screw-down crown and you never accidentally unscrew it, you're protected. If the crown isn't screwed in — even half a turn off — the rating doesn't apply.
04 · The gasketsTime destroys them.
The gaskets (O-rings) that seal the crown, caseback and crystal are almost always made of rubber, neoprene or Teflon. They degrade over time under heat, UV rays, pool chlorine and the chemicals we use daily — perfume, cream, sunscreen. A watch rated 10 ATM that has gone three years without maintenance might barely hold 3-5 ATM. When to have the seal checked:
- Every 2-3 years for watches used regularly in water (swimming, water sports).
- Every year for ISO 6425-certified dive watches.
- Immediately after a drop, a strong impact, or if the crown feels different (harder, softer) from when the watch was new.
- Before summer if you wear the watch at the beach — cheaper to check now than regret it later.
After every use in salt water: rinse the watch with cool fresh water, dry well with a soft cloth, paying attention to the crown area. Crystallized salt is abrasive and accelerates gasket degradation. If you wear the watch at the beach every summer, this 30-second step is worth as much as a full maintenance.
05 · WatchScopeThe timegrapher as early warning.
WatchScope doesn't test water resistance — that requires a pneumatic tool at a workshop. But it can give you an important early warning: if you've worn the watch in water and in the following days you notice an anomalous rate or a drop in amplitude, moisture may have entered the movement. What to watch for:
- Amplitude that collapses (from 260° to 190° within a few days): moisture inside the movement forms condensation on lubricated surfaces, increasing friction on the balance wheel. Act before rust damages the components.
- Very variable or suddenly high rate: moisture can alter the behaviour of the hairspring — effect similar to magnetization, but caused by physical condensation rather than magnetic fields.
- Before-and-after test: run a quick test before going to the beach and one after you get home. If parameters have changed significantly, take the watch to a watchmaker only — never open it yourself, to avoid introducing more humidity.
Never try to dry the watch yourself (no oven, no rice bag, no hair dryer). See a watchmaker within 24-48 hours: evaporating moisture leaves salt and mineral deposits that corrode metal surfaces. The sooner you act, the less damage there will be.
Frequently asked questions
What does 10 ATM mean on a watch?
10 ATM is a pressure rating: the case was tested to withstand the pressure of a 100-metre water column in a laboratory. It is a static-pressure figure, not a safe diving depth, because real movement, temperature swings and ageing gaskets all reduce it. In practice 10 ATM is comfortable for swimming and snorkelling, but not for scuba diving.
How many metres is 5 ATM?
One ATM equals roughly 10 metres of water column, so 5 ATM corresponds to 50 metres and 3 ATM to 30 metres on the test bench. Treat these as laboratory ratings rather than depths you should actually reach: a 5 ATM watch handles rain, hand-washing and a quick swim, but not high-impact water sports.
Is a 3 ATM watch waterproof, and can I swim or shower with it?
3 ATM (30 metres) only protects against splashes, rain and accidental immersion. It is not made for swimming, showering or any activity where water hits the case under pressure, and hot shower water can also degrade the gaskets. For swimming you want at least 5 ATM, and ideally 10 ATM.
Why does 10 ATM not mean I can dive to 100 metres?
The rating comes from a static test in still water, while diving and swimming add dynamic pressure from your movements that can briefly exceed the static figure several times over. Manufacturers publish the number as a safety margin, not as a maximum depth. As a rule of thumb, halve the figure on the dial for real-world peace of mind.
How can WatchScope help with water resistance?
WatchScope does not measure water resistance directly, but a mechanical watch that has taken on moisture often shows it first in its timing, as a sudden change in rate or amplitude after exposure to water or steam. By tracking your watch over time, WatchScope gives you an early warning to have the gaskets and seals checked before condensation reaches the movement.
Check the parameters
while it's still dry.
A quick test with WatchScope before and after the beach tells you in 30 seconds if anything has changed. Free on Android.