Watch Water Resistance Explained
Serdar D.Watch EditorA watch's water resistance figure is a static-pressure value measured in a lab, not the splashy reality of daily wear. Thirty metres handles rain, 100 covers swimming, and 200 is what serious divers look for, ideally with ISO 6425 (DIVER'S) certification for scuba. ATM and bar say the same thing, and the screw-down crown makes the difference. Below I explain what the figures really mean.
Key takeaways
- The water resistance figure is measured in a lab under static pressure, so read it one rung conservatively because real-world water moves.
- The ladder is clear: 30m for rain, 50m tolerates brief shallow swimming, 100m for genuine swimming, and 200m is what serious divers look for.
- ISO 6425 sets the certified dive-watch minimum at 100m; for scuba, look for the DIVER'S (ISO 6425) marking, not depth alone.
- ATM and bar are practically equal: 1 ATM is about 1 bar, roughly 10 metres of water, so 20 ATM means 200 metres.
- The screw-down crown is the most critical part of water resistance on most dive watches; always confirm it is fully closed before you get in the water.
- Whatever the figure, hot water and steam tire the seals, so the shower and sauna are best avoided at any rating.
What the water resistance number actually measures
The 100 metres printed on your dial does not mean you can take the watch down to 100 metres. That figure is measured in a lab under static pressure: it shows how much pressure the watch withstands while sitting perfectly still. In real life, water moves. An arm stroke, a jump into the sea, even holding the watch under a running tap spikes the momentary pressure well above the value on paper. So water resistance is a safety margin, not a depth promise. As a watchmaker, I tell people to read the figure one rung more conservatively than it sounds.
The 30, 50, 100, 200-metre ladder
Think of the figures as a practical ladder:
- 30 metres (3 ATM): Rain, hand washing, the occasional splash. No swimming.
- 50 metres (5 ATM): Tolerant of brief shallow swimming and hand washing, but not diving. Showers are best left for another time.
- 100 metres (10 ATM): Genuine swimming, snorkelling, the pool. This covers most daily needs, and it is also the minimum ISO 6425 sets for a certified divers' watch.
- 200 metres (20 ATM): What most serious divers look for in practice, with the DIVER'S (ISO 6425) marking ideally on top, since that certification adds shock, salt and overpressure testing on top of the depth figure.
This is exactly where a real dive watch separates from a dive-styled one. With a 200-metre depth rating and a screw-down crown, the Orient Mako 3 is a true 200-metre diver, ideal for swimming, snorkelling and skin diving. For scuba, look for an ISO 6425 (DIVER'S) certified model, which is tested to a stricter standard than the depth figure alone.
What ATM and bar mean
Sometimes the dial shows ATM or bar instead of metres. Both are pressure units, and in practice they are equal: 1 ATM is about 1 bar, roughly the pressure of a 10-metre column of water. So 20 ATM means 200 metres. For the finer technical distinction, see the ATM and bar entry. If you want the same genuine 200 metres in a smaller case, the Orient Mako 40 is a good example.
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Why the screw-down crown matters so much
The weakest point of water resistance is almost always the crown. An ordinary crown closes with nothing more than a push gasket and can leak under pressure. A screw-down crown threads into the case, sets a firm gasket seal and keeps water out. Most serious 200-metre watches use one, though it is not strictly required: gasketed push-pull crowns and crownless designs also reach 200 metres, because ISO 6425 asks the watch to pass the pressure test, not to use a particular crown type. Where a watch does have a screw-down crown, the single rule is this: make sure the crown is fully screwed down and closed before you get in the water. Going in with an open crown is the most common mistake people make with these watches. Tough-built watches like the classic crownless Casio G-Shock reach the same confidence by a different route, through sealed construction and a rugged case; note that several analogue G-Shock lines such as the Mudmaster, Frogman and Gulfmaster do use screw-down crowns, so that different route applies mainly to the crownless models.
What you can really do with each rating
A simple summary:
- Hand washing, rain: 30 metres is enough.
- Swimming, the pool, snorkelling: 100 metres is safe.
- Scuba and free diving: look for 200 metres, a screw-down crown and, for scuba in particular, an ISO 6425 (DIVER'S) certified model rather than depth alone.
Hot water and steam expand the seals, so whatever the figure, watchmakers advise against the shower and the sauna. Rinsing the watch in fresh water after the sea is a good habit too.
Care, the figure tires over time
The number on the dial is a factory value; it drops as the seals age. If you genuinely get the watch wet, have a pressure (water resistance) test done every few years, a check that takes only minutes. Ask for the seals to be renewed when the case is opened for a battery or service. To pick the right watch, see the beginner's watch guide, and for genuine divers, the best dive watches under £500.
Read the figure honestly, close the crown, test the seals. That is how water resistance lasts.
Specifications
| Specification | Value |
|---|---|
| Everyday swimming threshold | 100 metres (10 ATM) |
| ISO 6425 certified dive minimum | 100 metres |
| Recommended for serious diving | 200 metres (20 ATM) plus ISO 6425 |
| Unit equivalence | 1 ATM is about 1 bar, roughly 10 m |
| Critical part | Screw-down crown (on most dive watches) |
Pros
- Explains the real meaning of the figures and the static-pressure logic in plain terms
- Offers a practical decision ladder from 30 to 200 metres
- Clears up confusing terms like ATM, bar and the screw-down crown
- Backed by glossary terms and genuine 200-metre watches
Cons
- It is not a detailed head-to-head comparison of specific brand models
- It does not replace real dive training or a professional dive protocol
- It does not list every watch's exact ISO 6425 certification one by one
Verdict
Read the figure as a safety margin, not a depth promise. For daily wear, 100 metres is enough; if you take water seriously, the Orient Mako 3 with its screw-down crown and genuine 200 metres is a fine starting point for swimming, snorkelling and skin diving. For scuba, look specifically for an ISO 6425 (DIVER'S) certified model rather than depth alone. The Orient Mako 40 gives the same class for a slimmer wrist, and the Casio G-Shock suits anyone who wants absolute toughness. Whichever you pick, close the crown and have the seals tested periodically.
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Frequently asked questions
Can I dive with a watch rated 100 metres of water resistance?
A 100-metre rating is more than safe for swimming, the pool and snorkelling, but it is not designed for scuba diving. Real diving calls for 200 metres and a screw-down crown. The figure is measured under static pressure, so always read the depth conservatively.
What is the difference between ATM, bar and metres?
In practice they say the same thing. 1 ATM is about 1 bar, and that roughly equals the pressure of a 10-metre column of water. So 10 ATM means 100 metres and 20 ATM means 200 metres. Whichever one the dial shows, the logic does not change.
Why does water resistance decrease over time?
Water resistance is provided by the seals, and seals tire over time with heat and use. The figure on the dial is a factory value that drops as they age. If you get the watch wet, have a pressure test done every few years and renew the seals when the case is opened.
About the author
Serdar D.Watch Editor
View profileSerdar D. is the editor at BraveryWatch. He believes a good watch should be not just expensive but right. He gets deep into the details, then turns them into something that is genuinely a pleasure to read. He gives relaxed, useful advice through the eyes of someone who truly cares about watches.
