Water Resistance Ratings Explained: What 30m, 50m, 100m and 200m Really Permit

Water resistance is the most widely misread specification in watchmaking. The numbers look like depths, so people treat them as depths — and then wonder why a watch marked "30 metres" fogs up in the shower.

The figures are not depth ratings. They are the results of a static pressure test carried out in a laboratory on a new watch. Understanding that one distinction tells you nearly everything you need to know.

Why the numbers are not depths

A watch rated to 30 metres has been shown to withstand the static pressure equivalent to 30 metres of water — roughly 3 atmospheres — for a short period, in still water, at a stable temperature, when new.

Real water use is nothing like that. Diving into a pool, swimming a front crawl or holding a hand under a running tap all generate short-lived pressure spikes far above the static equivalent of your actual depth. Moving your arm through water at speed can briefly produce several atmospheres. That is why the safe use of a watch sits well below its printed rating.

Three other factors erode the margin:

Gaskets age. The rubber seals at the crown, case back and crystal harden and shrink over years. A ten-year-old watch does not hold its original rating unless it has been serviced and pressure-tested.

Temperature matters. Hot water expands metal and rubber at different rates, and steam penetrates where liquid water will not. A hot shower, a sauna or a hot tub is harder on a watch than a cool swimming pool, regardless of rating.

The crown is the weak point. A pushed-in crown seals adequately. A crown left half out — after setting the date, say — seals not at all. Screw-down crowns exist precisely to remove this failure mode.

You will also see ratings expressed in ATM or bar. One ATM is roughly equal to 10 metres, so 5 ATM means 50 metres and 20 ATM means 200 metres. They are the same figure written differently.

What each rating actually permits

The following is the conservative reading used across the industry. It is deliberately cautious, because the cost of being wrong is a ruined movement.

30 metres / 3 ATM — keep it dry

Splash resistance only. Rain, hand-washing spray, a spilled drink. Not showering, not swimming, not washing up.

This is the normal rating for dress watches, where a slim case and an exhibition case back matter more than sealing. Two good examples: the Tissot Everytime Swissmatic, a 40mm automatic just 11.6mm thick with a display case back, and the Tissot Le Locle in rose gold PVD at 39mm by 9.8mm. Both are rated to 30 metres, and in both cases that is a design decision rather than a shortcoming — you do not take a watch like this swimming.

The Bulova A-15 Pilot is also 30 metres, which is worth noting because it looks like a tool watch. Appearance is not a guide to sealing.

50 metres / 5 ATM — incidental water, not deliberate swimming

Hand-washing, rain, being caught in a downpour. Most manufacturers still advise against swimming. Where a watch has a screw-down crown, many owners swim in 50-metre watches without incident, but you are using up the margin rather than staying inside it.

Examples at this rating include the Citizen Urban Elegance BH5001-56L at 42mm and the Citizen Super Titanium AW1900-50L at 40mm, both rated 5 ATM.

100 metres / 10 ATM — swimming and snorkelling

This is the first rating at which ordinary water use becomes genuinely comfortable. Swimming, showering if the water is not scalding, snorkelling. Not scuba diving, and not high-speed water sports where impact pressure spikes.

One hundred metres is the workhorse rating and by far the best-represented figure in Tickmere's catalogue. The Casio Edifice Classic Date at 42mm and the Seiko 5 Sports SRPD61K1 at 42.5mm both sit here. For a watch that has to cope with an unplanned life, 100 metres is the sensible floor.

200 metres / 20 ATM — scuba diving, if it is a diver's watch

Two hundred metres covers recreational scuba diving, but the rating alone is not the whole story. A true diver's watch is built to the ISO 6425 standard, which requires more than pressure resistance: a unidirectional rotating bezel so elapsed time cannot be accidentally increased, legibility in darkness at 25cm, a clear indication that the watch is running, resistance to shock, magnetism and salt water, and strap security.

Watches that meet it say so. Seiko's Prospex divers are explicit about it — the SRPD21J1 at 45mm and the PADI-edition SRPE99 are both 200-metre pieces described as conforming to ISO standards and suitable for scuba diving, each running the 4R36 automatic with an approximately 41-hour reserve.

A 200-metre rating on a fashion watch without a rotating bezel or screw-down crown means the case passed a pressure test. It does not mean the watch is a diving instrument.

300 metres and beyond — saturation of the practical

Beyond 200 metres you are buying margin rather than capability, unless you dive professionally. That margin is not worthless: a watch rated to 300 metres has more headroom for aged gaskets and thermal shock than one rated to 100.

Tickmere's 300-metre pieces include the Tissot Seastar 1000, a 45.5mm quartz chronograph with a screw-down crown and unidirectional bezel; the Bulova Icon 98B316 with its screw-back case and screw-down crown; and the TAG Heuer Aquaracer Chronograph, a 43mm automatic with a black ceramic unidirectional bezel.

A quick reference

| Rating | Rain, splashes | Hand washing | Showering | Swimming | Snorkelling | Scuba | |---|---|---|---|---|---|---| | 30m / 3 ATM | Yes | Brief only | No | No | No | No | | 50m / 5 ATM | Yes | Yes | Not advised | Not advised | No | No | | 100m / 10 ATM | Yes | Yes | Yes | Yes | Yes | No | | 200m / 20 ATM | Yes | Yes | Yes | Yes | Yes | If ISO 6425 | | 300m+ | Yes | Yes | Yes | Yes | Yes | Yes |

Things that will defeat any rating

Some of the most common causes of water damage have nothing to do with the number on the dial.

• Operating the crown or pushers while wet. Never unscrew a crown, set the time or press a chronograph pusher with the watch in water. Chronograph pushers in particular are not sealed against actuation under pressure unless specifically designed to be.
• Leaving the crown unscrewed. On a screw-down crown watch, the seal only exists when the crown is fully home.
• Hot water and steam. Showers, saunas and hot tubs. Heat degrades gaskets faster than anything else and thermal cycling draws moisture in.
• Soap, shampoo and sun cream. Surfactants attack rubber seals and get into gasket channels. Rinse a watch in fresh water after swimming, particularly after the sea.
• Age. Gaskets are consumables.

Maintaining water resistance

If a watch matters to you and you use it in water, have the seals replaced and the case pressure-tested periodically — commonly cited as every two to three years for a watch in regular water use, and always after the case has been opened for a battery change. Any time a case back comes off, the rating is only as good as the reseal.

This is one of the quiet arguments for a light-powered movement. An Eco-Drive or solar watch never needs opening for a battery, so the seals stay undisturbed for as long as the watch runs. Our movements guide covers that trade-off in more detail, and the watch care guide sets out sensible service intervals.

If you are shopping specifically for water use, start with Tickmere's dive watches and read our guide to choosing a dive watch, which covers bezels, lume, crowns and strap security alongside the rating.

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