My most accurate watch – the Casio F-91W

casio-yellowJust for fun, I got the first quartz watch for my collection yesterday. I had to fork out £17 for it, and this Casio F-91W has so far kept perfect time in the last 24 hours.

I chose the yellow one, because I liked the colour best, and the main reason for the buy was to find out how accurate a super-cheap quartz watch is!

bin-laden-casioApparently, I’m not the only one who likes the value for money this watch gives you…
This lad in the photo on the right is apparently a huge fan as well! But he chose the boring black version, and look where that got him. Should have chosen yellow! His problem could also have been that he switched on the hourly beep function, and anyone turning that on should be shot …

The watch can show the time in either 12 hour am/pm or in a 24 hour display, and has a chronograph, calendar and an alarm on top – so pretty much everything you would want in a watch. The calendar doesn’t record the year, so it can’t deal with leap years, but I think I can live with that.

I’ll let it run for a couple of days and see if it stays well within the 1s/day accuracy Casio claims for the watch. So far, I expect it to do much better, e.g. <10s/month. Only one way to find out…


It’s a week later, and the Casio is 3/4s fast, so roughly 3-4 seconds per month, or 36 to 48s per year. Not a shabby performance for that price, I expected a lot worse. This can compete with a £2000 Omega quartz watch in accuracy. Actually, most owners of expensive quartz watches would be very happy with this performance. 😉

26 thoughts on “My most accurate watch – the Casio F-91W

  1. I found out that wearing the watch for a day and the watch started to gain time like a half a second then leaving it on the desk for a day it went back to perfect internet time . I guess its just the temperature but the thing keeps amazing time …And Argos seems to be the cheapest place to get it in black…..

  2. Compliments Christian for a recent and decent review of a timeless watch. This thing has been with us since 1991, and is still in production. Therefore, people still want it big time, year 2016. Well what else can you possibly say about a model. What most people don’t realize is:
    – quartz is as a rule many thousand times more accurate than pricewise mechanical movt.
    – quartz is indefinitely more PRECISE than a mechanical movt. It will always be off the ~same amount every day, upside down, all around, baked, frozen, shaken, anything but destroyed. Therefore it is predictable. And tough.
    – quartz crystals albeit almost the same, vary in characteristics as the manufacturing goes on. Some gain, some lose time. Thus, in the middle, it is not uncommon that a certain issue of watches turns out near atomically correct, off by a couple of seconds per… year! Effectively, you wouldn’t have to set this timepiece in your lifetime. For 20 bucks.

    So how durable is this watch? Well, unless you hit it with a hammer or dive with it and press the buttons for fun, it will last forever. Not the battery though. That only goes for like 10 years.

  3. I owned that Casio back in my long distance running days, in basic back. Looks like it hasn’t changed much. Very accurate, as you say, but not very durable if exposed to, ahem, violent shocks. 10 year battery life? Maybe if you never use any of the functions or the night light I seem to remember it having. Is yours still working?

    But the real question is, when one or more of the buttons stops working, or works only intermittently, or the display goes blank, can you service the movement?

    • Yes, mine is still working. I read a book about two guys rowing across the Pacific, and the only piece of kit that didn’t fail were their Casino watches. Probably the watch of choice for my single handed ocean crossing!

  4. The yellow color is genuine, it is available on some countries.
    I prefer the calendar not record the year than pre-program to 2099 in most Casio’s watches.

  5. As much as I dig vintage mechanical time pieces, I have a soft spot for these. I saved my paltry lawn mowing earnings in ’82 as a kid to buy one of these and was fascinated that it could run for five years on the same battery – which would have me through jr. high, high school and in college before it died. And they were right, it did. I lost mine in the woods by my fraternity house, didn’t notice the band breaking. Five years later I found it where countless cars had run over it on the new gravel driveway. Put a new battery and strap on it and now it’s my gym watch. Casio really earned their keep on this one though I wouldn’t be caught dead wearing a LCD watch anywhere besides the gym or a desert island nowadays.

    I appreciate your sense of humor re. Osama’s choice of timekeeping. Turns out this same watch has been a staple part of bomb timing for quite a while; quite an endorsement!

    • I was just going to comment to say this.

      Casio ONLY make black versions of this watch. Because it’s so popular there are loads of fakes. You’d think that such a cheap watch wouldn’t warrant producing fakes but they are so cheap to make and so easy to copy that there’s big business in it.

      You can tell if it’s fake by holding down the bottom right button for three seconds or so, it will show CA510 on the screen. Also if you hold all the buttons then every segment on the screen should display and stay displayed until you press another button.

      That said, even if your does this it just means it’s a “good” fake since Casio only make them in black.

      • Also, my yellow F91W passed both tests. It displays CASIo when I press the right button, and it shows all symbols on the display if I press all three buttons…
        I know this story is widely peddled on watch forums, but that’s not a decent source of information. So my yellow F91W is real, even though you claim that Casio never made the watch in any other colour but black? (sounds like Henry Ford has risen from the dead…)

  6. The adverts at the side of your blog must spy on you…….there is an advertisment for the same watch in a salmon pink shade for £8.75.
    This Casio could become your favourite watch 😉

    • Yes, Google Adsense is very good at knowing what you have looked at – sometimes a bit scary…

      I admit I’ve been totally had paying £17 for mine, but I just completely forgot to check on eBay! I bought it at, and I’ve learned my lesson 😉

  7. £17?? Blimey – they saw you coming… you could probably buy the factory for that 😉

    Apparently there are “fake” versions of this now that have changeable back-light colours…

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