Water resistance ratings are the most widely misread specification in watches. A watch marked 50M is not safe to take 50 metres underwater. Understanding what the numbers mean will stop you ruining a watch.
Why the numbers mislead
Water resistance is tested in a laboratory under static pressure — the watch sits still in a pressure chamber. Real use is dynamic. Swinging your arm through water, diving in, or turning on a shower jet all create pressure spikes far above the static equivalent of your actual depth.
The rating is therefore a pressure specification, not a depth permission.
What each rating actually permits
| Rating | Safe for | Not safe for |
|---|---|---|
| No rating / "splash" | Rain, handwashing | Showering, swimming |
| 30M (3 bar) | Rain, handwashing | Showering, swimming, diving |
| 50M (5 bar) | Showering, shallow swimming | Diving, water sports, hot water |
| 100M (10 bar) | Swimming, snorkelling | Scuba diving, high-speed water sports |
| 200M (20 bar) | Swimming, snorkelling, most water sports | Saturation / mixed-gas diving |
"200M" and "20 bar" are the same thing
Casio marks some models in metres and some in bar. They are equivalent: 10 metres of water is approximately 1 bar of pressure. A watch marked 20 BAR is a 200M watch. There is no difference in capability implied by the choice of unit.
Where common Casio lines sit
- F-91W, A158, A168 — splash resistant only. Not for swimming.
- MQ-24 and similar standard analogs — typically 50M or lower. Check the specific reference.
- Most Baby-G — typically 100M.
- Most G-Shock — 200M.
- Frogman — ISO-rated dive specification, the only G-Shock line built to a genuine diving standard.
Three things that quietly destroy water resistance
Heat
Hot showers, saunas and hot tubs are worse than cold water at the same depth. Heat expands the case and degrades gaskets, and steam penetrates seals that would hold against liquid water. Casio explicitly advises against wearing most models in a sauna.
Pressing buttons underwater
Unless the model is specifically rated for it, pushing a pusher underwater breaks the seal at exactly the moment pressure is applied. This is one of the most common causes of water ingress.
Age
Gaskets are rubber. They compress, dry out and lose elasticity over years. A ten-year-old 200M watch that has never been serviced is not a 200M watch any more.
Practical rule
If you want to swim, buy 100M or better. If you want to not think about it at all, buy 200M. If a watch has no explicit depth marking, treat it as splash resistant only regardless of what the seller says.
Every reference in our Casio collection lists its water resistance rating on the product page.
Frequently asked questions
► Can I shower with a 50M watch?
Generally yes for brief, warm — not hot — showers, but it is not ideal. Heat and soap degrade gaskets faster than cold water. A 100M or 200M watch is a safer choice if you shower with it daily.
► Is 20 bar the same as 200M?
Yes. Ten metres of water is roughly one bar of pressure, so 20 bar and 200M describe the same specification.
► Can I swim with a 30M watch?
No. A 30M rating covers rain and handwashing only.
► Does water resistance last forever?
No. Gaskets age and lose elasticity. An older watch that has never been serviced will not hold its original rating.
