Casio F-91W

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Casio F-91W
Casio F-91W 5051.jpg
Casio F-91W-1 watch with a resin case and resin strap

The Casio F-91W is a digital watch manufactured by Japanese electronics company Casio. Introduced in 1989[1] as a successor of the F-87W,[2] it is popular for its low price and long battery life.[3] As of 2011, annual production of the watch is 3 million units, which makes it the most sold watch in the world.[4] Its ubiquitous use in the construction of timers for terrorist bombs prompted US officials to view the wearing of these watches as a "sign of al-Qaeda".[5][6]



Designed by Ryusuke Moriai as his first design for Casio,[7] the case of the F-91W measures 37.5 by 34.5 by 8.5 millimetres (1.48 by 1.36 by 0.33 in). The case is primarily made of resin,[8] with a stainless steel caseback and buttons with the manufacturer's module number, 593, stamped on the caseback. The resin strap is 18 millimetres (0.71 in) at the fitting and 22 mm across the widest part of the lugs. The total weight is 21 grams (0.74 oz).


The F-91W is a chronograph, featuring a 1100 second stopwatch with a count up to 59:59.99 (nearly one hour). The stopwatch also has the feature to mark net and split time (lap). Other features include an hourly time beep and a single daily alarm lasting 20 seconds and an annual calendar, with adjustment for leap years not supported as the watch does not record the year. February is always counted as 28 days.[9] The watch uses a faint, green LED backlight located to the left of the display for illumination (in earlier versions it was an amber microlight). According to manufacturer estimates, the watch is reported to be accurate to ±30 seconds per month.[9]

The quartz movement, designated Module 593, is powered by a single CR2016 3-volt lithium button cell.

Water resistance[edit]

The watch front is marked "Water Resist",[9] but Casio reports different values for different variants of the watch. The black version (F91W-1)[10] is "30 meter / 3 bar" (i.e. 100 feet / 44 psi), the ISO standard meaning of which is: "Suitable for everyday use. Splash/rain resistant. NOT suitable for showering, bathing, swimming, snorkeling, water related work and fishing".


Casio F-91W, in regular timekeeping mode and using the 24-hour display option. The watch is currently set to sound the alarm and hourly chime

The watch is controlled by three side-mounted push-buttons.

  • The upper left button, labeled "Light", turns on the light, cancels the alarm, resets the stopwatch or marks the split (lap) time, and is used for selecting settings.
  • The lower left button, labeled "Mode", cycles the modes of the watch: time display, alarm, stopwatch, and time/date adjustment.
  • The right button, labeled "Alarm On-Off/24hr", is the function button: when used, it starts and stops the stopwatch, changes the settings currently being adjusted, or switches between the 12- and 24-hour modes, depending on what mode the watch is currently in. Pressing all three buttons at the same time will fill all the cells on the LCD until any button is pressed again.[11]

The time or date is adjusted by pressing the lower left button three times to bring the watch to time adjustment mode. The top left button is used to cycle through seconds, hours, minutes, month, date, day, and normal mode. The right button is used to adjust the flashing value displayed. Unlike any other value, the seconds can only be zeroed. Should this happen before 30 seconds, the watch will zero in at the beginning of the current minute. After 30 seconds it will start the next minute as displayed. When the adjustments are finished, the bottom left button can be pressed once to return the watch to normal mode.

The watch display shows the day of the week, day of the month, hour, minute, seconds, and the signs PM in the afternoon – or 24H (24-hour clock) – at all times, the alarm signal status (bar of vertical lines), and the hourly signal status (double beep on the hour, shown as a bell) are present when activated in the alarm mode.

In stopwatch mode, minutes, seconds, and hundredths of a second are shown.

Usage in terrorism[edit]

This improvised timer for a time bomb was captured in the early 2000s.

According to secret documents issued to interrogators at Guantanamo Bay, obtained[12] and released by The Guardian, "the Casio F-91W digital watch was declared to be 'the sign of al-Qaeda' and a contributing factor to continued detention of prisoners by the analysts stationed at Guantanamo Bay. Briefing documents used to train staff in assessing the threat level of new detainees advise that possession of the F-91W and the A159W – available online for as little as £4 – suggests the wearer has been trained in bomb making by al-Qaeda in Afghanistan."[13] United States military intelligence officials have identified the F-91W as a watch that terrorists use in constructing time bombs.[14][15][16][17]

This association was highlighted in the Denbeaux study, and may have been used in some cases at Guantanamo Bay.[18] An article published in The Washington Post in 1996 reported that Abdul Hakim Murad, Wali Khan Amin Shah, and Ramzi Ahmed Yousef had developed techniques to use commonly available Casio digital watches to detonate time bombs.[19] Casio watches were mentioned almost 150 times in prisoner assessments from Guantanamo.[20]

On July 12, 2006, the magazine Mother Jones provided excerpts from the transcripts of a selection of the Guantanamo detainees.[16] The article informed readers:

More than a dozen detainees were cited for owning cheap digital watches, particularly "the infamous Casio watch of the type used by Al Qaeda members for bomb detonators."

The article quoted Abdullah Kamel Abdullah Kamel Al Kandari:

When they told me that Casios were used by Al Qaeda and the watch was for explosives, I was shocked... If I had known that, I would have thrown it away. I'm not stupid. We have four chaplains [at Guantanamo]; all of them wear this watch.


Model name Description
F-91-WC Neon colours: orange, blue, green, pink, beige and yellow[21]
F-91-WM Metallic coloured cases
F-84W Japanese Domestic Market (JDM) model. Uses the same 593 module, but the case's design is more similar to the older F-28W and F-87W. It has lugs instead of an embedded strap.
F-94W Circular icon arrangement in the display
W59 Waterproof up to 50 metres
A158W, A159W, A163W, A164W Stainless steel band
A159WGEA-1 Stainless steel band, gold colour
F-105W, A168W, A168WG, A168WEGM, A168WEM, W-86 Equipped with electroluminescent backlighting instead of the LED backlight in other variants. Available in black colour with a resin strap or in silver or gold colours with a stainless steel bracelet. Slightly different icon arrangement on the display and a thicker case due to the backlight system. Some versions also feature a negative display
LA680 A smaller variant, marketed towards women
F-91-WS Translucent colours: blue, pink, white, and grey[22]
F-91W-3, F-91WG-9 Similar to the original F-91W, but including a green and gold trim respectively on the face.[23]


Holding the right button for 3 seconds in the main timekeeping mode leads the display to show "CASIo", which is useful to spot a counterfeit model (applicable for newer models of the F-91W and its variants, including F-94W and A158W)

Counterfeits of this watch are very common, despite its low price tag. These counterfeits generally have a lower plastic build quality, narrower LCD viewing angles, louder and higher-pitched beeps, and significantly less accurate timekeeping than genuine models.[24] The newer modules with the green LED light can be tested by holding the right button for over 3 seconds in the main timekeeping mode; this will lead the display to show "CASIo", as a test for authenticity. With the advancement in technology, however, some counterfeit models have also been developed to show this sign, although these are fairly uncommon. This leaves the only method of distinguishing them as assessing the overall build quality, timekeeping accuracy, display viewing angle and the printing on the screen.[24][25]


  1. ^ a b Moyer, Phillip (June 15, 2019). "The case of an iconic watch: how lazy writers and Wikipedia create and spread fake 'facts'". KSNV. Retrieved June 19, 2019.
  2. ^ Casio: General Catalog (1984): The Casio Collection. A Superb Selection of Modern Timepieces
  3. ^ "Casio F91W-1: The retro watch with a strange double life". Montredo. February 4, 2020. Retrieved April 19, 2020.
  4. ^ "Casio F-91W – the classic quartz digital watch". ICON (published August 19, 2011). August 19, 2011. Archived from the original on July 29, 2019. Retrieved August 6, 2019.
  5. ^ "Guantánamo Bay files: Casio wristwatch 'the sign of al-Qaida'". The Guardian. April 25, 2011.
  6. ^ "Casio F-91W: The strangely ubiquitous watch". BBC NEWS. April 26, 2011.
  7. ^ Ariel Adams (April 16, 2017). "An Afternoon In Tokyo With The Man Who Designs Casio G-Shock Watches". [A Blog To Watch. Retrieved July 6, 2019.
  8. ^ "F-91W-1 | Digital | Youth | Timepieces | CASIO". CASIO Official Website. Retrieved February 16, 2022.
  9. ^ a b c "F91W-1 Classic Timepiece". Casio. Retrieved April 27, 2011.
  10. ^ "F-91W-1XY". Casio. Retrieved May 22, 2013.
  11. ^ "Manual" (PDF). Casio. Retrieved November 26, 2014.
  12. ^ "Guantánamo files: How interrogators were told to spot al-Qaida and Taliban members". the Guardian. April 25, 2011. Retrieved April 10, 2019.
  13. ^ James Ball (April 25, 2011). "Guantánamo Bay files: Casio wristwatch 'the sign of al-Qaida'". The Guardian. London.
  14. ^ "USA v. al Qahtani" (PDF). US Department of Defense. November 7, 2005. Retrieved February 27, 2007.
  15. ^ "Combatant status review board for Mohammed Ahmad Said el Edah". The New York Times. October 6, 2004.
  16. ^ a b "Why Am I in Cuba?". Mother Jones. July 12, 2006.
  17. ^ "Summary of Evidence memo (.pdf) prepared for Sabri Mohammed Ebrahim Al Qurashi's Combatant Status Review Tribunal – page 216" (PDF). October 13, 2004. Archived from the original (PDF) on July 31, 2006.
  18. ^ "Empty Evidence". National Journal. February 3, 2006. Archived from the original on July 4, 2008.
  19. ^ R. Jeffrey Smith (July 21, 1996). "New Devices May Foil Airline Security". The Washington Post. pp. A01. Retrieved March 14, 2008.
  20. ^ Batya (July 9, 2020). "How did the Casio F91W Become a Terrorist Icon?". Reaper Feed.
  21. ^ "Casio Collection | Timepieces | Products | CASIO". Retrieved June 8, 2019.
  22. ^ "Casio International | Timepieces | Digital | F-91WS-2 | CASIO". Retrieved April 8, 2020.
  23. ^ "F-91WG-9 | VINTAGE SERIES | Timepieces | CASIO". CASIO Official Website. Retrieved December 12, 2021.
  24. ^ a b "Comparing a genuine Casio F91-W with a fake". July 2018. Retrieved March 25, 2019.
  25. ^ Nguyen, Khoi (August 31, 2018). "Casio F91W Review - Retro Digital Sport Watch". Gentleman Within (review). Retrieved August 28, 2019.

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