How to spot a fake Casio F-91W
What to check before buying or modding an F-91W, and why showing CASIo on the screen doesn't mean you've got a real one.
The Casio F-91W is the world’s most popular watch, and with that comes people creating fakes and counterfeits. Some of them even pass the button test you’ve probably seen online.
If you’re buying one to wear or take apart for a mod, I’d check a few things before reaching for a screwdriver. Start with where it came from, then look at the printing, finish and display. You want those details to agree with each other. No single marking or button combination will settle it.
I’m focusing on the black F-91W-1, with the blue border around the screen. If you have another colour or version, pull up photos of that exact model.
First, where did you buy it?
Buying directly from Casio or a verified authorised retailer saves a lot of this detective work. On a marketplace, look at who’s actually selling the watch. Casio branding at the top of the page doesn’t tell you who will send it to you.
For a used one, ask for photos of the actual watch: front, back and strap. A receipt helps, and so does knowing whether anything has been replaced. A genuine watch with an aftermarket strap won’t match every detail in a photo of a new one.
I wouldn’t put much weight on the price either. Cheap is a reason to check, but there’s nothing stopping someone charging full price for a fake.
Look closely at the printing and plastic
Open Casio’s product photos and compare the lettering, the borders around the LCD and the button labels. Look for misspellings, sloppy edges, uneven spacing or labels sitting in the wrong place. Don’t get too caught up in a slightly different shade of blue; lighting and phone cameras can do that.
Then look around the case edges, strap holes and the little keeper that holds the loose end of the strap. Leftover bits of plastic from moulding, rough edges and an uneven finish are worth a closer look. This owner’s side-by-side comparison shows those differences on a genuine watch and one they identified as fake. It’s useful to see an example, though other fakes won’t necessarily have the same faults.
You’ll also find advice about the tiny U below the face printing, country markings, screw heads and boxes. I’d be careful with rules built around just one of those. You need to know which version you’re comparing, and on a used watch you may be looking at replacement parts. A box can be copied too.
On the back, check the model and module number. The familiar F-91W uses module 593, which you can look up in Casio’s manual search. That gets you the right instructions. It doesn’t authenticate the watch; the number can be copied along with the rest of the engraving.
The CASIo trick works on fakes too
Hold the right-hand button for a few seconds while the watch is showing the time, and the familiar CASIo lettering should appear. This is often passed around as a quick way to tell whether an F-91W is real.
Unfortunately, copies can do it too. In the comparison linked above, both watches showed the lettering. Both also turned on all the LCD segments when all three buttons were pressed together.
Try it if you’re curious, but don’t let a seller use it as proof. If it doesn’t work, check that you’re in the time display and that the button works normally before drawing a conclusion.
Spend a minute using the watch
Look at the screen straight on in ordinary room light, then tilt it a little. Can you read it comfortably? Are unused parts of the digits showing through as strong shadows? Comparing it beside a known genuine, unmodified F-91W helps here. Use the same angle and light for both.
Try the light in a dim room as well. Brightness alone won’t tell you much about authenticity, especially with an old battery or a modified display.
Next, work through the controls using Casio’s F-91W manual:
- A short press of the right button on the time screen switches between 12-hour and 24-hour time.
- The lower-left button cycles through the alarm, stopwatch and time-setting screens, then back to the time.
- The stopwatch should start, stop, reset and handle split times.
- In alarm mode, holding the right button lets you hear the buzzer.
If the controls behave differently from the manual, that’s worth following up. But a broken watch can still be a real Casio. Missing digits or a button that stopped working after someone opened it may be an assembly problem; the display troubleshooting guide covers those faults.
What about weighing it?
Casio gives the standard F-91W-1 a weight of 21 g and case dimensions of 38.2 × 35.2 × 8.5 mm.
If yours is a long way off, look into it. I wouldn’t worry about a small reading difference on a kitchen scale, and there’s little point comparing weights once you’ve changed the strap or case. A fake can match the weight and size, so this is only another clue.
Leave the screwdriver until later
If you’ve just bought it and something looks wrong, take photos and contact the seller before opening it. It’s easier to show what arrived when you haven’t taken it apart.
The module inside can reveal differences that aren’t obvious from the outside, but you’ll need a reliable reference for the same module. A battery brand or an unfamiliar sticker won’t tell you enough on its own.
This matters for modding too. Before starting the F-91W background mod, sort out any doubts about the donor watch. Afterwards, the display and possibly other parts won’t look stock, which makes these comparisons harder. Ask what was changed if you’re buying someone else’s finished build.
If you’re still unsure, keep the listing and receipt, photograph the details that bother you, and record any odd behaviour. Get in touch with the seller while you can still return it, or ask Casio support what they can check in your region. I’d rather sort that out before spending an evening on a mod.