Casio F-91W

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Casio F91W
Casio F-91W 5051.jpg
Type Quartz
Display Digital
Introduced 1991[1]

The Casio F91W is a quartz digital watch, manufactured by the Japanese company Casio Computer Co., Ltd. It was introduced in 1991.[1] The watch is available in several variants. Casio does not release sales figures for the watch, but says the watch continues to sell well.[1]


The F91W has a 1/100 second stopwatch with a count up to 59:59.99 (almost one hour) and measuring modes of net time, split time, and 1st and 2nd place times. There are the options of an hourly time beep and a single daily alarm. It has an automatic calendar, although auto-adjustment for leap years is not supported, as the watch does not record the year.[2] The watch uses a green LED backlight to illuminate the display. The watch is reported to be as accurate as ±30 seconds per month[2] (6 minutes per year) by the manufacturer.

The watch is powered by a single CR2016 lithium button cell, which Casio claims will last approximately seven years (assuming 20 seconds of alarm and one second of light usage per day). The watch case measures 37.5 × 33.5 × 9.5 mm and weighs 20 g. The manufacturer's module number for this model (stamped on the stainless steel rear of the watch case) is 593.

Water resistance[edit]

The watch front is marked WATER RESIST.,[2] but Casio reports different values for different variants of the watch. The black version (F91W-1XY)[3] 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.", but the colored versions (e.g. F-91WC-2AEF) are "DIN 8310 / ISO 2281- resistant to minor splashing".


Casio F-91W, in daily alarm mode and using the 24-hour display option. The watch is currently set to sound its daily alarm at 7:30 a.m., but hourly beeps are disabled.

The watch is controlled by three side-mounted pushbuttons. The upper left button turns on the backlight, cancels the alarm, and is used for selecting settings. The lower left button cycles the modes of the watch: time display, alarm, stopwatch, and time 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.[4]

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. 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 or 24H (24-hour clock), the alarm signal status, and the hourly signal status (double beep on the hour).

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

Claimed use in terrorism[edit]

This improvised timer for a time bomb was captured in the early 2000s[where?].

According to secret documents issued to interrogators at Guantánamo Bay, obtained[5] 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 Guantánamo Bay. Briefing documents used to train staff in assessing the threat level of new detainees advise that possession of the F91W – available online for as little as £4 – suggests the wearer has been trained in bomb making by al-Qaeda in Afghanistan."[6] United States Military intelligence officials have identified the F91W as a watch that terrorists use when constructing time bombs.[7][8][9][10][11]

This association was highlighted in the Denbeaux study, and may have been used in some cases at the Guantanamo Bay detention camp.[12] 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.[13]


F105W, a popular variation of the F91W.

There are many variants of the F91W that have the same specifications and operational features, including the Casio F105W, which uses an electroluminescent backlight instead of an LED to illuminate the display, the Casio A158W, A159W, A168W, LA680, and B640, with stainless steel bands, and the W59 and F94W.


  1. ^ a b c Denise Winterman (April 26, 2011). "Casio F-91W: The strangely ubiquitous watch". BBC News magazine. Retrieved April 27, 2011. 
  2. ^ a b c "F91W-1 Classic Timepiece". Casio. Retrieved April 27, 2011. 
  3. ^ "F-91W-1XY". Casio. Retrieved May 22, 2013. 
  4. ^ "Manual" (PDF). Casio. Retrieved November 26, 2014. 
  5. ^
  6. ^ James Ball (April 25, 2011). "Guantánamo Bay files: Casio wristwatch 'the sign of al-Qaida'". The Guardian (London). 
  7. ^ "USA v. al Qahtani" (PDF). US Department of Defense. November 7, 2005. Retrieved February 27, 2007. 
  8. ^ "Combatant status review board for Mohammed Ahmad Said el Edah". The New York Times. October 6, 2004. 
  9. ^ "Why Am I in Cuba?". Mother Jones. July 12, 2006. 
  10. ^ "Guantanamo Captives Jailed Because Of Digital Watches". WCVB. March 10, 2006. 
  11. ^ "Summary of Evidence memo (.pdf) prepared for Sabri Mohammed Ebrahim Al Qurashi's Combatant Status Review Tribunal – page 216" (PDF). October 13, 2004. 
  12. ^ "Empty Evidence". National Journal. February 3, 2006. 
  13. ^ R. Jeffrey Smith (July 21, 1996). "New Devices May Foil Airline Security". Washington Post. pp. A01. Retrieved March 14, 2008. 

External links[edit]