Just How Water Resistant Is The Casio F91W?

Water resistance is an important feature of a modern watch. It makes wearing the watch far more practical in this modern world of sudden rainstorms and urban water balloon ambushes. The Casio F91W, one of the company’s most popular watches, is claimed to be water resistant to “30 meters”, which in ISO parlance, means it is suitable for splashes and rain resistance only. [Rostislav Persion] wanted to get a better idea of what this really meant, so set about investigating for himself.

The first step was to simply immerse the watch under 5.5″ of cold tap water while pressing the buttons and observing for any signs of water ingress. Already, the watch proved it is far more than just rain resistant, so [Rotislav] decided to disassemble the watch and learn how it achieved this.

Disassembly revealed that the watch’s case was entirely sealed, except for three buttons. The buttons, however, are specially designed in order to seal with the plastic case of the watch. Each button consists of a stainless steel pin, machined to be larger on the outside-facing side than the inside. The buttons also have a rubber O-ring seal to allow them to move in the case without allowing water to leak inside. [Rotislav] then compares the simple design to buttons used on watches with higher water resistance ratings, which boast multiple O-ring seals and more complex designs.

Given [Rotislav’s] results, we’d be far more confident getting our affordable Casio watches a little wet. Obviously, we wouldn’t expect to make a warranty claim if damage occurred from use outside the specs, but it’s clear the watch is far more capable than standards might suggest. If that’s not enough though, you can always set about modifying the watch to improve its water resistance even further.

24 thoughts on “Just How Water Resistant Is The Casio F91W?

  1. But the question still remains. How water resistant is the casio F91w. Down to 10metrs? 15meters? 30meters? 50meters? I need answers now that you have asked the question!

      1. A cheap watch is easy to test, swim as deep as you can, and if it survives, it’s okay. If it dies, no real loss.

        but a destructive test on an expensive watch isn’tas easily replaced.

  2. The big M doesn’t mean meters. It’s just tested up to a static pressure of 3 Bars or 30 m of hydrostatic pressure.

    At that pressure, the seals will hold as long as nothing moves and the watch isn’t bumped or shaken.

    1. Yeah but that practically means “swimming to basically 30M”. The change in pressure from being moved around on your wrist violently is basically trivial compared to the pressure.

  3. My G-Shock says “water resist 20 bar”.

    My Waveceptor from about 2006 says 10 bar (it’s still on its first battery, but has a solar charger).

    My fancy Casio with the thermometer and altitude says 10 bar.

    I thought they were marked in Metres. I guess the advertising is where I thiught I saw depth.

    I’ve seen warnings about watches after the battery’s been changed, the integrity not as good. I suspect using the buttons underwater might be risky.

    Myfirst Casio thirty years agowas low end. I never wore it swimming, but I never took it off while bathing or washi g dishes. For most of us, thst’s waterproof enough.

  4. I’m surprised people still buy the F91W. I understand the appeal of these watches, but the F91W has a really horribly weak backlight. For 2 bucks more you can get the F105W which has this satisfying crystal clear blue EL backlight.

    Even in the 80s when I bought my first F105W I thought the F91W was already obsolete :)

    1. huzzah_a_man_of_quality.meme

      Although when I lost my F105W I decided to replace it with an A168W (same guts), slightly snazzier and I prefer the metal band. Similar button construction as the F91W here but with the world’s tiniest E-clips retaining them from popping out.

      The F91W is slightly smaller in all dimensions than the F105W/A158W/A168W, so there is that. But they could probably have slimmed the F91W down even more by deleting the useless light. Casio must still have a warehouse of dim LEDs from the 90s they’re trying to use up.

    2. I’ve always considered the weak light to be a feature. Completely adequate for reading the time at night (i. e. in bed), but guaranteed not to wake whoever you happen to share that bed with. Also great outside because it doesn’t destroy your night vision.

      On the other hand, a dedicated button for the light would have been great. Try setting the alarm time in the dark, it’s maddening…

    3. A blue backlight may have been a feature 10 years ago, but now I just consider it icky. Something like an orange backlight is much more practical. After driving a car with a stupidly effective transflective orange LCD I consider that superior to almost anything else. Very legible in direct sunlight, great during nighttime and everything in between.

    1. Yeah this didn’t test much.
      Ages ago, I went for a dive using my a Timex Ironman digital watch, I can’t remember what exactly it was rated for, but I think it was 100m / 300 ft. IIRC after about 30 feet, the pressure is sufficient that the buttons do not work any more- they all get pushed in at the same time because the pressure outside easily overcomes the small amount of atmospheric pressure inside the watch. So I guess it is technically correct that depth and submersion didn’t damage the watch, but it was functionally useless. I suspect that would be the case for this Casio in the article.

      A proper dive watch is a whole ‘nother thing.

    1. Did they jostle or vibrate the watches? Equivalent tends to be not really equivalent when realistic dynamic pressures are involved. Just moving or tapping the watch under pressure can mean the difference between pass and fail.

  5. Ah yes, the perennial confusion around watch water resistance.

    1. Yes the “m” is for metres, no it’s not reliable
    2. “Dynamic pressure” is not the reason

    A watch’s water resistance in many cases is entirely notional. It’s pretty rare to see a cheap watch marked “ISO 2281”. The manufacturer estimates a pressure that it estimates there is an acceptably low probability of failure and calls it a day. Even under ISO 2281 testing is only a lot sample procedure. This is why the recommended use scenarios are absurdly conservative, it’s not that going for a swim can generate more than 3 bars of dynamic pressure, it’s that the 3 bar rating is a guideline not a guarantee.

    Because of this, it isn’t uncommon to see individual watches exceed their rated pressures, and also to fail well below their rated pressures, even in the same make and model for cheaper watches..

    Some manufacturers do fo course design and test their watches to a high standard, particularly when they are intended and marketed specifically as a dive watch. These comply with ISO 6425 and are marked accordingly

    1. Nailed it.

      Water resistance claims on most watches, unless they’re specifically designed for use by saturation divers and the like, are more suggestions than rules. More often than not manufacturers grossly underestimate water resistance for warranty purposes. Water resistance ratings for desk divers is for show anyway. Do you really need a Rolex Sea Dweller when all you do is snorkel with the kids once or twice a summer? No.

      The truth is that for most people 50 to 100 meters is more than adequate for daily use. I wear a Casio MDV-106 every day and am more than confident that it’s claimed 200 meter WR is plenty. Honestly, I’ll break my arm before I can inflict a hard enough knock to break the watch and most likely drown long before it springs a leak.

      If you’re scuba diving you’re going to get a watch that is actually tested to deal with the depths you intend to use it in.

  6. I think of the F-91W as the VW Combi Van of watches. Not a lot to it, but it has a unique charm.

    I gave one to my nephew when he was 6 and it has survived over four years of the kind of abuse that only a bit his age can inflict on a a wearable device. Not only that, but it looks almost as good as new.

    That’s some bang for your buck, folks

  7. When I was a kid (late 80’s to early 90’s), I used to go swimming with my watch still on my wrist, only ones claiming to be water resistant of course, and never had any problems with it. Probably also never got below 10ft either.

  8. Longing for pulverized silicon – an ode to the F91W By Robbie

    In the same year we traded a pop-culture icon turned president for a former vice president, the US experienced one of its worst stock market crashes in history, the Shanghai virus was a top news story, and Michael Keaton debuted as Batman. The Blade Runner decade defined an era of techno-utilitarianism. Auto emission controls eased the edges of environmental effects, while auto designs squashed curvaceous bodies to unsightly boxes, reminiscent of our modern bit-slinging silicon squares now cluttering the desktops in mazes of cubicles. In cruelty, the core digital element that shackles us to power and networks also dominates the composition of the sun drenched shoreline we long to visit during our evening session of Sweatin’ to the Oldies. We seek a reminder, a hint that our lives are meant to be lived beyond a wall of interlinked cross-spreadsheet functions; a close, consistent reminder that though silicon rules our lives, it’s a life spent on pulverized silicon dioxide we desire.

    Today, that reminder is available in the form of an efficient, sport-minded style that could have only been brought by that decade. A quartz-driven, power sipping silicon marvel of 1989. The digital watch that has outsold every other one in history. A watch that the only thing smart about it is its looks: The Casio F-91W.

  9. Great post! I’ve always wondered about the water resistance of the Casio F91W. It’s such a popular watch, and knowing how well it can withstand water is essential. I appreciate the effort you put into explaining the watch’s water resistance rating and the real-life scenarios you tested it in. It’s impressive to see that the F91W performs exceptionally well even beyond its claimed 30-meter water resistance. Thanks for sharing your findings and helping us understand the watch better!

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