TL;DR
- The Casio F-91W is a compact digital watch released in June 1989, successor to the F-87W, and the best-selling watch in the world with over 100 million units sold.
- Runs on module 593, the same module used across all 18 color versions from 1989 to present.
- Battery is a CR2016, cheap and easy to find anywhere. Rated 7 years, often lasts 10 or more.
- Crystal is acrylic, scratches easily but polishes out with Polywatch or cerium oxide.
- 18 color versions across four generations: classic black with accent rings (1989–2003), full monochrome WC series (2010), metallic two-tone WM series (2018), and translucent WS series (2020).
- Mod-friendly: popular mods include NATO strap swaps, negative LCD filter, LED backlight upgrade, and the Ollee Watch module that turns it into a quasi-smartwatch.
- Counterfeits are common. Hold the A button for 3 seconds in timekeeping mode — a genuine watch shows “CASIo” on the display.
- Close relatives worth knowing: A158W (steel bracelet), A168W (better backlight), G-Shock DW-5600 (same DNA, built tougher).
- Serious collectors wear it deliberately next to Rolexes and Omegas. The ultimate anti-flex flex.
Casio F-91W – Technical Data & Model Information
The Casio F-91W is a compact digital watch released in June 1989 as the successor to the F-87W. It is the best-selling watch in the world, with Casio producing 3 million units per year as of 2011 and total sales passing 100 million pieces. The F-91W runs on module 593, which drives the LCD display, alarm, stopwatch, and calendar functions. The resin case, acrylic crystal, and 21-gram build made it a global daily wearer from day one.
The model evolved visually across four generations while keeping module 593 throughout. The F-91W-1 started in 1989 with a black resin case and blue accent ring. The WC series introduced full monochrome colors in 2010. The WM series added metallic cases with black straps in 2018. The WS series brought fully translucent resin in 2020, showing the module through the shell.
For collectors and restorers, the F-91W is a solid starting point. The CR2016 battery is cheap and widely available. The acrylic crystal scratches over time but polishes out easily with Polywatch or cerium oxide. The stainless steel caseback carries the module 593 stamp, making identification straightforward during any restoration. 🔧
Casio F-91W – Quick Technical Data
| Field | Data |
|---|---|
| Model | Casio F-91W |
| Module | 593 |
| Battery | CR2016 |
| Manual | Casio F-91W (593) Manual (PDF) |
| Glass Type | Acrylic |
| Strap Size | 18 mm (lug) / 22 mm (case) |
| Color Versions | The Casio F-91W comes in 18 color versions, released across four generations: classic black with colored accents — 3 versions (1989–2003), full monochrome — 6 versions (2010), two-tone with metallic case and black strap — 5 versions (2018), and transparent — 4 versions (2020). |
| Functions | Time display (12/24hr format) Date & day display Daily alarm (auto-sounds 20 sec/day) Hourly time signal Stopwatch (59:59.99 max, 1/100s precision) Stopwatch modes: normal, split, net time, 1st–2nd place EL backlight Auto-calendar (auto February 28-day correction) |
| Production Years | 1989 – present |
How many color versions does the Casio F-91W have?
The Casio F-91W has 18 color versions, released across four generations: classic black with colored accents (3 versions, 1989–2003), full monochrome WC series (6 versions, 2010), two-tone metallic WM series (5 versions, 2018), and transparent WS series (4 versions, 2020).
The classic black versions of the F-91W
The first three F-91W versions share the same black resin case and strap. The only difference between them is the color of the inner accent ring around the dial.
F-91W-1 (1989) — Black with Blue Accent Ring
The F-91W-1 launched in 1989 as the original. It has a black resin case and strap with a blue inner ring around the dial, yellow model text, and red arrow markers.
F-91WG-9 (1997) — Black with Gold Accent Ring
The F-91WG-9 arrived in 1997 with the same black body as the original, but swaps the blue ring for a gold one. The model text also turns gold, and the side hardware matches in gold tone.
F-91W-3 (2003) — Black with Dark Green Accents
The F-91W-3 came out in 2003 as the most colorful of the three. It pairs a green inner ring with a yellow accent border, orange model text, and orange arrow markers.
The monochrome WC versions of the F-91W
In 2010, Casio released six WC (Wide Color) series versions. Each one is built in a single color from case to strap, with tone-on-tone dial text throughout. Simple, bold, and surprisingly wearable.
F-91WC-2A (2010) — Light Blue Monochrome
The F-91WC-2A goes fully light blue across the case, bezel, and strap. Silver-tone side hardware is the only contrast on the whole watch.
F-91WC-3 (2010) — Green Monochrome
The F-91WC-3 uses a bold bright green on the case, bezel, and strap. White dial text is the only break in the color.
F-91WC-4A (2010) — Pink/Raspberry Monochrome
The F-91WC-4A covers the case, bezel, and strap in bright pink/raspberry. White dial text keeps it readable.
F-91WC-4A2 (2010) — Orange Monochrome
The F-91WC-4A2 goes full orange on the case, bezel, and strap. White dial text is the only contrast.
F-91WC-8A (2010) — Cream/Beige Monochrome
The F-91WC-8A is the quietest WC version. It uses a soft cream/beige across the whole watch, with tone-on-tone dial text that almost disappears into the body.
F-91WC-9A (2010) — Yellow Monochrome
The F-91WC-9A applies a warm yellow/mustard tone to the case, bezel, and strap. White/cream dial text provides a subtle contrast.
The two-tone metallic WM versions of the F-91W
In 2018, Casio released five WM (Wide Metallic) series versions. Each one pairs a metallic-finish case with a black resin strap and a black dial surround. The contrast between the shiny case and the dark strap gives these a noticeably more polished look than the earlier versions.
F-91WM-1B (2018) — Metallic Grey Case
The F-91WM-1B uses a metallic grey case and bezel on a black strap. White dial text sits cleanly against the black dial surround.
F-91WM-2A (2018) — Metallic Blue Case
The F-91WM-2A puts a metallic blue case and bezel on a black strap. White dial text keeps the contrast sharp.
F-91WM-3A (2018) — Metallic Dark Green/Olive Case
The F-91WM-3A uses a metallic dark green/olive case and bezel on a black strap. It is the darkest and most subtle colorway in the WM series.
F-91WM-7A (2018) — Metallic Silver Case
The F-91WM-7A goes with a metallic silver case and bezel on a black strap. Clean, classic, and easy to wear with anything.
F-91WM-9A (2018) — Metallic Gold Case
The F-91WM-9A uses a metallic gold case and bezel on a black strap. The dial text also turns gold to match the case, giving it the most consistent finish in the WM series.
Transparent WS versions of the F-91W
In 2020, Casio released four WS series versions. Each one uses fully translucent resin for the case, bezel, and strap. The internal module is visible through the shell, which is a fun detail for anyone who likes to see how things work inside. 👍
F-91WS-2 (2020) — Translucent Light Blue
The F-91WS-2 uses a clear light blue resin for the case, bezel, and strap. The dial text follows the same light blue tone throughout.
F-91WS-4 (2020) — Translucent Pink
The F-91WS-4 covers the case, bezel, and strap in clear pink resin. The dial text matches the pink tone of the body.
F-91WS-7 (2020) — Translucent White/Silver
The F-91WS-7 uses a near-clear resin with a subtle silver-grey tint on the case, bezel, and strap. Grey tone-on-tone dial text keeps the whole watch very minimal.
F-91WS-8 (2020) — Translucent Grey
The F-91WS-8 is the darkest WS version. It uses grey-tinted translucent resin across the case, bezel, and strap, with grey tone-on-tone dial text to match.
Physical Design of the Casio F-91W
The F-91W has a compact resin case with three side buttons and an acrylic crystal. It sits small on the wrist, which is part of its charm. The dial runs a classic two-row LCD layout, showing the time on the main row and the date, day, and secondary data on the smaller row above. The resin strap is integrated into the case, keeping the whole profile low and lightweight.
Strap & Fit of the Casio F-91W
The F-91W uses a lightweight resin strap across all versions, making it a solid daily wearer. The lug width is 18 mm and the case width is 22 mm, which is useful to know when hunting for replacement straps. The WM series adds a two-tone twist, pairing a metallic case with a black resin strap — a small detail that defines the whole WM look and is worth preserving during any restoration.
Casio F-91W and Terrorism
The F-91W earned a strange reputation in the early 2000s. The watch’s alarm can be set beyond 24 hours, which made it easy to adapt as a timer for improvised explosive devices. Al-Qaeda reportedly distributed the watches to recruits at training camps in Afghanistan and Pakistan throughout the 1990s. (Watches of Espionage)
The most documented case is Ramzi Yousef, who used an F-91W timer in 1994 to detonate an explosive on Philippine Airlines Flight 434, killing a Japanese tourist. (Watches of Espionage)
At Guantanamo Bay, the F-91W appeared nearly 150 times in prisoner assessment documents. (NPR) Briefing documents used to train Guantanamo staff stated that possession of the F-91W suggested the wearer had been trained in bomb making by al-Qaeda. (Wikipedia) One detainee responded directly: “We have four chaplains at Guantanamo — all of them wear this watch.” (Wikipedia)
The watch was cheap, available everywhere, and worn by millions of ordinary people. The controversy never hurt its reputation. Today the F-91W sits in collections next to Seikos, G-Shocks, and even Rolexes. ⌚
Mod Culture of the Casio F-91W
The F-91W has one of the most active mod communities of any watch at this price point. The mods range from simple strap swaps to full module replacements. 🛠️
Strap swaps are the most popular starting point. The stock 18 mm resin strap is fine, but many collectors replace it with a NATO strap for a cleaner, more customizable look. NATO straps fit the 18 mm lug width perfectly and come in hundreds of colors and patterns, which makes the F-91W surprisingly versatile for such a cheap base watch.
LCD mods are where things get more creative. The most popular options are:
- Negative polarizing filter — flips the display to white digits on a dark background, completely changing the look of the watch
- Fluorescent foil behind the screen — makes the display glow constantly without pressing the backlight button
- Colored or graphic screen filters — change the LCD color or add a pattern overlay
Some collectors like (lukebuildswatches) run small businesses doing custom screen mods to order, so you can pick the exact color or graphic you want for your dial.
LED backlight upgrades are also common, where collectors boost the brightness of the stock green LED for a stronger glow in the dark.
The most advanced mod is the Ollee Watch, a drop-in replacement module that turns the F-91W into a quasi-smartwatch. For $54.99 DIY or $99.99 pre-modded, it adds Bluetooth connectivity, step tracking, heart rate monitoring, ambient temperature, world time, alarm snooze, sunrise/sunset tracking, and a customizable LED backlight. Installation takes about 6 minutes with two small screwdrivers and tweezers — no soldering needed. The Ollee Watch is compatible with any watch running module 593, including the F-91W, A158W, F-84W, A159W, W-59, A171W, and A163W.
Casio F-91W in Watch Collector Culture
The F-91W is often called the “Nokia 3310 of watches.” The design has not changed in over 35 years, it runs the same module 593 from 1989, and it costs less than a decent lunch.
The numbers speak for themselves. Casio produces 3 million units per year, and total sales since 1989 have passed 100 million pieces. No hype, no waiting lists. Just a watch people keep buying decade after decade.
It weighs 21 grams and the battery lasts 7 years rated, but many collectors report 10 years or more before needing a swap. The watch appeared in Stranger Things, Squid Game, and on Ryan Reynolds’ wrist in Free Guy. Barack Obama wore one before his presidency.
The real story in collector culture is simpler. The F-91W sits in collections next to Rolexes and Omegas on purpose. Serious collectors wear it deliberately next to watches worth thousands. A $10 resin watch holding its own next to the most expensive pieces in the hobby. Every collector who wears it knows exactly what they are doing. ⌚
Close relatives worth knowing: the A158W offers the same soul in a stainless steel bracelet, the A168W adds better backlight, and the G-Shock DW-5600 is the F-91W on steroids
Conclusion
The Casio F-91W is a digital resin watch released in June 1989, running module 593, weighing 21 grams, and selling over 100 million units worldwide. The design has not changed in 35 years. The CR2016 battery outlasts its rated 7 years. The acrylic crystal polishes out with Polywatch. It costs less than most watch straps. There is not much else to say about the specs, and that is the point.
For collectors, the F-91W covers everything. The 18 color versions across the F-91W-1, WC, WM, and WS series give plenty to hunt. The 18 mm lug width fits NATO straps directly. The negative polarizing filter, fluorescent foil mod, and Ollee Watch module upgrade keep the platform alive for years. The stainless steel caseback stamps module 593 for easy identification during any restoration. It sits in collections next to Rolexes and Omegas on purpose, worn deliberately by collectors who know exactly what a $10 watch can say. ⌚
Sources & Further Reading
The information in this article was gathered through hands-on research for CasioRestore.com. Sources include direct examination of real F-91W watches across multiple versions, the official Casio module 593 manual, Casio’s own product pages, and knowledge shared by collectors on Reddit (r/casio, r/casiomods), Instagram, and collector forums.
Additional references used during research:
- Wikipedia — Casio F-91W article, release date correction and citogenesis documentation
- The Guardian — leaked Guantanamo Bay briefing documents referencing the F-91W
- NPR — Guantanamo prisoner assessment reporting, April 2011
- WikiLeaks — “Matrix of Threat Indicators for Enemy Combatants” document, 2011
- Watches of Espionage — documented cases of F-91W use in IED construction
- KSNV News 3 Las Vegas — original investigation correcting the 1989 release date
- PacParts Inc. — Casio America parts supplier, confirming 1989 release date
- Ollee Watch — official product page, specifications and compatibility list
- lukebuildswatches (Instagram) — custom screen mod reference
- Reddit r/casio, r/casiomods — negative filter mod, fluorescent foil mod, and community mod documentation
Casio F-91W – Frequently Asked Questions
When was the Casio F-91W released?
The Casio F-91W was released in June 1989 as the successor to the F-87W.
What module does the Casio F-91W use?
The Casio F-91W runs on module 593, which drives the LCD display, alarm, stopwatch, and calendar functions across all 18 color versions.
What battery does the Casio F-91W use?
The Casio F-91W uses a CR2016 3V lithium battery, rated for 7 years but often lasting 10 years or more in practice.
How many color versions does the Casio F-91W have?
The Casio F-91W has 18 color versions across four generations: 3 classic black versions with accent rings (1989–2003), 6 monochrome WC series versions (2010), 5 metallic two-tone WM series versions (2018), and 4 transparent WS series versions (2020).
What is the strap size of the Casio F-91W?
The Casio F-91W has an 18 mm lug width and a 22 mm case width, which means standard 18 mm NATO straps fit directly without any modification.
Can you polish the crystal on a Casio F-91W?
Yes. The Casio F-91W uses an acrylic crystal, which scratches easily but polishes out with Polywatch or cerium oxide during a restoration.
Is the Casio F-91W water resistant?
The Casio F-91W is rated at 30 meters / 3 bar, making it suitable for splashes and rain but not for swimming or bathing.
How do I spot a fake Casio F-91W?
On newer versions, hold the A button for 3 seconds in timekeeping mode. A genuine Casio F-91W displays “CASIo” on the screen. Fake versions usually have louder beeps, narrower LCD viewing angles, and lower quality plastic compared to the original.
What mods can you do to the Casio F-91W?
Popular Casio F-91W mods include NATO strap swaps using the 18 mm lug width, negative polarizing filter to flip the LCD display, fluorescent foil behind the screen for constant glow, colored screen filters, LED backlight upgrades, and the Ollee Watch module replacement which adds Bluetooth, step tracking, and heart rate monitoring for $54.99 DIY.
Why do serious watch collectors wear the Casio F-91W?
Serious collectors wear the Casio F-91W deliberately next to much more expensive watches like Rolexes and Omegas. At $10, 21 grams, and 35 years of unchanged design, it represents the purest form of Casio minimalism and holds its own in any collection.
Daniel is the person behind Vintage Casio Restore, a project dedicated to restoring and documenting vintage Casio digital watches from the 80s and 90s. Based in Poland, he combines technical precision with a passion for retro design, bringing forgotten models back to life and sharing their stories online.Outside the workshop, Daniel works in a digital agency, managing SEO and content strategy for technology clients, blending his professional experience with his love for classic tech and design.
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