Dive Watch Ratings Explained: What 100m, 200m and ISO 6425 Really Mean

A water-resistance rating printed on a dial or caseback looks like a depth limit, but it is not. A watch marked 100m has been tested to withstand a pressure equal to that depth under laboratory conditions, with the watch sitting still. Real life adds motion, temperature changes and ageing seals, which is why a rating is a margin of safety rather than a promise.

As a practical guide, 30m and 50m ratings mean splash resistance: fine for rain and hand washing, not for swimming. 100m is suitable for swimming and snorkelling. 200m and above is where most proper dive watches begin, and is more than recreational scuba diving needs. Crowns and pushers also matter: a screw-down crown is a much stronger seal than a push-in one.

ISO 6425 is the international standard for diver's watches. A watch that is marked as meeting it has to clear a longer list of tests than water pressure alone. It must have a unidirectional rotating bezel with minute markings, so an accidental knock can only shorten your dive time, never lengthen it. It must be legible in the dark from a distance, show that it is running, and withstand shocks, magnetic fields, thermal changes and salt water. Not every good dive watch is certified, and some brands test to their own stricter rules, but the standard is a useful checklist.

Helium escape valves and very deep ratings such as 1000m or more are built for saturation diving, where divers live in pressurised chambers. They are interesting engineering, but a recreational diver gains nothing from them, and they add thickness and weight.

When comparing divers on our watches page, look at the water-resistance figure alongside the crown type, bezel type and case thickness. A slim 200m watch with a screw-down crown and a unidirectional bezel will suit most people better than an oversized 1000m model.

Finally, seals age. If you swim with your watch, have the gaskets checked and pressure-tested about every two to three years, and always rinse it in fresh water after the sea.