Water Resistance
Thirty metres does not mean thirty metres. Here is what each rating actually permits.
At your own risk
On the edge of the rating. Heat, pressure changes and soap all work against the gaskets.
Rain, washing up and an accidental dunk. Not a swimming watch.
| Rating | Rain | Shower | Swimming pool | Snorkelling | Scuba diving | Hot tub or sauna |
|---|---|---|---|---|---|---|
| Not water resistant | At your own risk | No | No | No | No | No |
| 3 ATM · 30 m | Yes | No | No | No | No | No |
| 5 ATM · 50 m | Yes | At your own risk | At your own risk | No | No | No |
| 10 ATM · 100 m | Yes | Yes | Yes | At your own risk | No | No |
| 20 ATM · 200 m | Yes | Yes | Yes | Yes | At your own risk | No |
| 30 ATM · 300 m or more | Yes | Yes | Yes | Yes | Yes | At your own risk |
✓ within the rating · ~ at your own risk · ✕ past what it covers. Ratings are tested on new gaskets in still water; heat, steam and soap all shorten them.
What 30, 50, 100 and 200 metres actually let you do
A water resistance rating is not a depth. It is the result of a static pressure test, carried out in a laboratory, on a new watch with new gaskets, in still water at a steady temperature. None of those conditions describes a swimming pool, and that is why a watch marked 30 metres should never go anywhere near one.
The useful way to read the marking is as a list of permissions rather than a number. This instrument does that, and reads everything one step down when the seals have not been seen for years — because gaskets, not the case, are what keep water out.
Why 30 metres does not mean thirty metres
Movement multiplies pressure. Swinging an arm through water, or diving off the side of a pool, briefly generates far more pressure on the case than standing still three metres under would. The test does not simulate any of that, so the industry convention is to treat 30 m as splash resistance, 50 m as resistance to accidental immersion, and 100 m as the first rating you can actually swim in.
Temperature does the rest of the damage. Hot water and steam make gaskets expand and then contract, and a shower or a hot tub is a harsher environment for a watch than a cold pool. That is why almost nothing short of a proper dive watch belongs in a sauna.
Gaskets are the rating
Water resistance is a consumable. The rubber or silicone seals at the crown, case back and crystal harden, flatten and shrink with age, heat and the oils on your skin. A watch rated to 200 metres in 2009 and never opened since is not a 200 metre watch today.
A pressure test costs very little and takes minutes. Have one done at every service, and again before any trip where the watch is going in the sea. Almost every water damage claim is a gasket that was never checked.
The habits that matter more than the rating
Whatever the number on the case back says:
- Screw the crown fully down before water. An unscrewed crown defeats any rating instantly.
- Never operate pushers, the crown or a chronograph while the watch is submerged.
- Rinse salt water and chlorine off the case and bracelet with fresh water afterwards.
- Keep leather straps out of water entirely — the watch may survive; the strap will not.
- Treat steam, hot tubs and saunas as off limits unless the watch is a certified diver.
What ISO 6425 adds
A watch marked simply "200 m" and one marked "200 m Diver's" are not the same thing. The word diver's indicates compliance with ISO 6425, which requires individual testing rather than batch sampling, resistance to a stronger overpressure, legibility in darkness, a unidirectional elapsed-time bezel and resistance to magnetism, shock and salt water. If you actually dive, that word is the one to look for.
| Marking | Rain & washing | Shower | Swimming | Snorkelling | Scuba |
|---|---|---|---|---|---|
| 3 ATM · 30 m | Yes | No | No | No | No |
| 5 ATM · 50 m | Yes | At your risk | At your risk | No | No |
| 10 ATM · 100 m | Yes | Yes | Yes | At your risk | No |
| 20 ATM · 200 m | Yes | Yes | Yes | Yes | At your risk |
| 30 ATM · 300 m+ | Yes | Yes | Yes | Yes | Yes |
Assumes a screw-down crown where fitted and gaskets in good order. On an unserviced watch, read every column one step down.
Questions we are asked
- Can I swim with a 50 m water resistant watch?
- It is not recommended. Fifty metres covers rain, hand washing and accidental immersion; swimming generates pressure spikes the rating was never tested against. One hundred metres is the first rating suitable for swimming.
- Can I shower with a 100 m watch?
- Usually yes, with the crown screwed down, but hot water and soap are hard on gaskets and shorten their life. Many watchmakers would rather you took it off.
- What does 5 ATM mean on a watch?
- Five atmospheres of static pressure, equivalent to 50 metres of depth in a laboratory test. In practice it means splashes and accidental immersion, not swimming.
- How often should water resistance be tested?
- At every service, and at least every two years on a watch you swim with. Gaskets degrade with heat, age and skin oils whether the watch is worn or not.
Before the WaterRead the watch itself
Photograph it and the instrument will tell you what it appears to be — useful before you decide what it can survive.