Water Resistance in Watches: What Those Numbers Actually Mean

If you have ever looked at the back of a watch and seen markings such as 30M, 50M, 100M, 10 ATM or 20 ATM, you may have wondered what they actually mean.

At first glance, the numbers look straightforward. A watch marked 100M might appear to mean that it can simply be taken to a depth of 100 metres.

In practice, it is not quite that simple.

Water resistance ratings are based on controlled testing conditions. They are useful for comparing watches, but they should not be treated as a literal indication of how deep you can safely take a watch underwater in everyday use.

Understanding the ratings makes it much easier to choose the right watch for swimming, water sports or general daily wear.

What Do Metres, ATM and Bar Mean?

Watch manufacturers commonly use three different ways of expressing water resistance:

• Metres (M), such as 30M, 50M or 100M
• ATM, meaning atmospheres
• Bar, another unit used to express pressure

For everyday comparison:

1 ATM is approximately equal to 1 bar and around 10 metres of water pressure.

That means:

• 3 ATM is roughly equivalent to 30M
• 5 ATM is roughly equivalent to 50M
• 10 ATM is roughly equivalent to 100M
• 20 ATM is roughly equivalent to 200M

These figures describe the level of pressure resistance demonstrated during testing. They should not be read simply as recommended operating depths.

Why a 100M Watch Does Not Simply Mean 100 Metres Underwater

Water resistance testing is carried out under controlled conditions.

A new watch may be tested at a specified pressure while stationary and with its seals, case, crystal and crown in good condition.

Real world use is different.

The water resistance of a watch can be affected by factors such as:

• The age and condition of its seals and gaskets
• Whether the crown is fully closed or screwed down
• Previous servicing or battery replacement
• Damage to the case or crystal
• Exposure to salt water or chemicals
• Changes in temperature
• General wear over time

For that reason, the manufacturer's guidance for the individual watch should always take priority over the number printed on the dial or case back.

Common Water Resistance Ratings Explained

The table is a general guide only. Two watches with the same stated water resistance may have different intended uses, depending on their design and manufacturer.

Is a 30M Watch Waterproof?

No watch should really be thought of as permanently waterproof.

A watch marked 30M or 3 ATM is normally designed to cope with light everyday exposure such as rain or accidental splashes.

It should not automatically be assumed to be suitable for swimming.

If you regularly expect your watch to be submerged, it is sensible to choose a model with a higher water resistance rating and check what the manufacturer specifically allows.

Can You Swim With a 50M Watch?

Many watches rated to 50M or 5 ATM are intended to cope with swimming.

However, this should still be checked against the manufacturer's instructions for the particular model.

A 50M rating should not automatically be taken to mean that the watch is suitable for activities such as snorkelling, diving or high impact water sports.

What Does 100M Water Resistance Mean?

A watch rated to 100M or 10 ATM generally offers a much higher level of water resistance than a 30M or 50M watch.

It is commonly suitable for:

• Swimming
• Snorkelling
• Surface water activities
• General use around water

That does not necessarily make it a diving watch.

If you intend to scuba dive, look for a watch specifically designed and specified for diving rather than relying only on its metre rating.

What About 200M Watches?

A 200M or 20 ATM rating indicates a high level of water resistance.

These watches are often designed for more demanding use around water, but a 200M marking alone should not automatically be treated as proof that the watch is suitable for scuba diving.

For diving, check whether the watch is specifically described as a diver's watch and follow the manufacturer's stated limits.

What Is an ISO 6425 Diver's Watch?

ISO 6425 is an international standard covering watches designed specifically for diving.

A diver's watch is subject to requirements beyond a simple pressure resistance figure.

Depending on the watch and certification, these can include features relating to:

• Water resistance
• Visibility
• Timing indication
• Resistance to external forces
• Reliability under diving conditions

If you intend to use a watch for scuba diving, a properly specified diver's watch is a more meaningful indicator than the metre rating alone.

Can You Shower With a Water Resistant Watch?

This is one area where caution is sensible.

Even if a watch is water resistant, manufacturers may give different advice about showering.

Hot water, steam, soap and sudden temperature changes can all place additional demands on seals and gaskets.

If you are unsure, removing the watch before showering is the safer option.

Never Operate the Crown Underwater

As a general rule, avoid:

• Pulling out the crown underwater
• Adjusting the time underwater
• Operating pushers underwater unless the manufacturer specifically states that the watch is designed for this

Opening or moving components connected to the case can affect the seal.

For watches with a screw down crown, make sure the crown is fully secured before exposure to water.

How to Maintain a Watch's Water Resistance

Water resistance can reduce over time as seals age and the watch is exposed to normal wear.

A few simple precautions can help maintain it.

Have the Water Resistance Checked

If a watch is regularly used for swimming or other water activities, consider having its water resistance checked periodically.

It is particularly sensible after:

• A battery replacement
• Case opening or servicing
• Damage or a hard impact
• Several years of regular use

Rinse After Salt Water

After using a suitable water resistant watch in the sea, rinse it with fresh water.

Salt deposits can accumulate around the case, crown and bracelet.

Be Careful With Chlorine

Pool water contains chemicals that can affect some materials over time.

Rinsing the watch with fresh water after swimming is a sensible precaution.

Check the Crown

Before exposing the watch to water, make sure the crown is properly closed.

If the watch has a screw down crown, ensure it is securely screwed into position.

A Quick Water Resistance Guide

A simple way to think about water resistance is:

30M / 3 ATM
Suitable mainly for splashes and everyday exposure.

50M / 5 ATM
Commonly suitable for swimming.

100M / 10 ATM
Commonly suitable for swimming, snorkelling and general water activities.

200M / 20 ATM
Higher level water resistance for more demanding use, subject to the manufacturer's guidance.

Diver's Watch
Specifically designed and tested for diving.

Does Water Resistance Last Forever?

No.

Water resistance depends on the condition of components such as:

• Gaskets
• Seals
• Crown
• Crystal
• Case back

These components can deteriorate with age and use.

A watch that was water resistant when new may not retain exactly the same level of protection indefinitely, particularly if it has been opened, damaged or has not been serviced for a long time.

The Bottom Line

Water resistance ratings are useful, but they should be treated as guidance rather than literal diving depths.

The number on the watch tells you about the level of pressure resistance it was designed and tested to withstand, but real world suitability depends on the watch's design, condition and the manufacturer's instructions.

As a general rule:

• Use lower rated watches for everyday exposure and splashes
• Choose a higher rating if you regularly swim
• Check the manufacturer's guidance before using a watch for more demanding water activities
• Use a purpose designed diver's watch if you intend to scuba dive
• Have the water resistance checked periodically if the watch is regularly exposed to water

Choosing the right level of water resistance means thinking about how you intend to use the watch, rather than simply choosing the biggest number on the dial.