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The Casio F-91W is a digital watch manufactured by Japanese electronics company Casio. Introduced in 1989,[1] it is popular for its simplicity, reliability, and unpretentious clean design. Due to this reason, it is still in production with the same design.[2] Annual production of the watch is 3 million units per year.[3] The F-91W is a part of the Casio F-series watches.

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



Designed by Ryusuke Moriai as his first design for Casio,[4] the case of the F-91W measures 37.5 by 33.5 by 8.5 millimetres (1.48 by 1.32 by 0.33 in) and weighs 21 grams (0.74 oz). The case is primarily made of plastic, 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 22mm across the widest part of the lugs.


The F-91W has a ​1100 second stopwatch with a count up to 59:59.99 (nearly one hour). The stop watch also has the feature to mark net and split time (lap). Other features include an hourly time beep (hourly chime) and a single daily alarm lasting 20 seconds and an automatic calendar, although auto-adjustment for leap years is not supported as the watch does not record the year. February is always counted as 28 days.[5] 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.[5]

The quartz movement is powered by a single CR2016 3-volt lithium button cell, which lasts at least seven years,[citation needed] assuming 20 seconds of alarm and one second of light usage per day. However, these are seldom used in practice, so it is not uncommon for the battery to last 10 years or longer.

Water resistanceEdit

The watch front is marked WATER RESIST,[5] but Casio reports different values for different variants of the watch. The black version (F91W-1)[6] is "30 meter / 3 bar", 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 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 cycles the modes of the watch: time display, alarm, stopwatch, and time/date adjustment. The button on the right 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.[7] 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 terrorismEdit

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

According to secret documents issued to interrogators at Guantanamo Bay, obtained[8] 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."[9] United States Military intelligence officials have identified the F-91W as a watch that terrorists use in constructing time bombs.[10][11][12][13]

This association was highlighted in the Denbeaux study, and may have been used in some cases at the Guantanamo Bay detention camp.[14] 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.[15]

On July 12, 2006, the magazine Mother Jones provided excerpts from the transcripts of a selection of the Guantanamo detainees.[12] 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 Abudallah Kamel:

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[16]
F-91-WM Metallic coloured cases
F-94W Circular icon arrangement in the display
W59 Waterproof up to 50 metres
A158W, A159W, A163W, A164W Stainless steel band
A159WGA-1 Stainless steel band, gold colour
F-105W, A168W, A168WG, A168WEGM, A168WEM Equipped with electroluminescent backlighting instead of the LED incandescent 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


Pressing the right hand button for 3 seconds in the main timekeeping mode leads the display to read "CASIo" which is useful to spot a counterfeit (applicable for newer models of the F-91W and its variants)

The watch is worn all over the world. Its good design, high durability, low price and user friendly simplicity all contribute to this phenomenon. It has become a fashion statement and some variants have also become "collector items" in the watch community. "It is a modest masterpiece", says design critic Stephen Bayley.[2]


Counterfeits, which are quite common although the price tag of the original is very low, generally have a slightly lower plastic build quality, the LCD viewing angles are not as wide, and are significantly less accurate than the original.[17] The newer modules with the green LED light can be tested by pressing the right button for over 3 seconds in the main timekeeping mode; this action would lead the display to read "CASIo", as a test for authenticity. With the advancement in technology, however, counterfeit watches have also been developed to show the "CASIo" sign. This leaves the only method of distinguishing them as assessing the overall build quality, time keeping, display and the printing on the screen.[17][18]


  1. ^ 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. ^ a b Denise Winterman (April 26, 2011). "Casio F-91W: The strangely ubiquitous watch". BBC News magazine. Retrieved April 27, 2011.
  3. ^ "Casio F-91W – the classic quartz digital watch". ICON. Retrieved August 6, 2019.
  4. ^ 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.
  5. ^ a b c "F91W-1 Classic Timepiece". Casio. Retrieved April 27, 2011.
  6. ^ "F-91W-1XY". Casio. Retrieved May 22, 2013.
  7. ^ "Manual" (PDF). Casio. Retrieved November 26, 2014.
  8. ^ "Guantánamo files: How interrogators were told to spot al-Qaida and Taliban members". the Guardian. April 25, 2011. Retrieved April 10, 2019.
  9. ^ James Ball (April 25, 2011). "Guantánamo Bay files: Casio wristwatch 'the sign of al-Qaida'". The Guardian. London.
  10. ^ "USA v. al Qahtani" (PDF). US Department of Defense. November 7, 2005. Retrieved February 27, 2007.
  11. ^ "Combatant status review board for Mohammed Ahmad Said el Edah". The New York Times. October 6, 2004.
  12. ^ a b "Why Am I in Cuba?". Mother Jones. July 12, 2006.
  13. ^ "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.
  14. ^ "Empty Evidence". National Journal. February 3, 2006. Archived from the original on July 4, 2008.
  15. ^ R. Jeffrey Smith (July 21, 1996). "New Devices May Foil Airline Security". The Washington Post. pp. A01. Retrieved March 14, 2008.
  16. ^ "Casio Collection | Timepieces | Products | CASIO". Retrieved June 8, 2019.
  17. ^ a b "Comparing a genuine Casio F91-W with a fake". July 2018. Retrieved March 25, 2019.
  18. ^ Nguyen, Khoi (August 31, 2018). "Casio F91W Review - Retro Digital Sport Watch". Gentleman Within (review). Retrieved August 28, 2019.

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