Casio F-91W

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Casio F-91W
Casio F-91W digital watch.jpg
A Casio F-91W-1 watch with a plastic case and resin strap
Manufacturer Casio
Type Quartz
Display Digital
Introduced 1991[1]

The Casio F-91W is a quartz digital watch model, manufactured by the Japanese company Casio Computer Co., Ltd. It is a part of the Casio F-series of digital watches. The original F-91W was introduced in 1991 and is still in production and demand.[1] It is popular for its simplicity, reliability, and unpretentious, clean design. Its design has not changed since it was first brought out over 20 years ago.[2] The watch is available in several variants. Casio does not release sales figures for the model, but admits it continues to sell "well".[1]


The F-91W has a ​1100 second stopwatch with a count up to 59:59.99 (nearly one hour). Marking net and split time (lap) is also featured. There are the options of an hourly time beep and a single daily alarm lasting 20 seconds. It has 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.[3] The watch uses a faint, green LED backlight located to the left of the display for illumination. The watch is reported to be accurate to ±30 seconds per month by Casio.[3]

The watch is powered by a single CR2016 3-volt lithium button cell, which lasts at least seven years, 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.

The watch case 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 manufacturer's module number for this model (stamped on the stainless steel rear of the watch case) is 593. The strap is 18mm at the fitting and 22mm across the widest part of the lugs.

Water resistance[edit]

The watch has a reliable rubber seal between its steel back and case, but does not have any seals for its buttons. Since they fit tight enough, surface tension prevents the water from getting inside, but only to a certain point of water pressure. Therefore, the watch front is marked WATER RESIST,[3] but Casio reports different values for different variants of the watch. The black version (F91W-1)[4] 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". Even though the black version will resist an occasional water submersion if the buttons are not pressed, the watch should not be used in such conditions on a regular basis.


The watch is known to be very durable, people have bought the watch to use in a variety of situations: such as outdoors activities, working as a mechanic, doing construction jobs, or as a soldier. The body of the watch is known for lasting longer than the 10 year battery life. The watch will also outlast the plastic strap that it comes with. Casio replacement straps are typically about the same price as a new F91W watch, so people usually buy cheaper knock-off F91W's online from websites such as AliExpress and harvest the straps from those watches to do a strap replacement when the time comes.


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 push-buttons. The upper left button turns on the backlight, 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. In time mode, pressing this button for three seconds will make the display read "CASIo" if the watch has genuine parts or ones produced under legitimate license. Pressing all three buttons at the same time will fill all the cells on the LCD until any button is pressed again.[5]

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 lines), and the hourly signal status (double beep on the hour; a bell).

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[6] 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 – available online for as little as £4 – suggests the wearer has been trained in bomb making by al-Qaeda in Afghanistan."[7] United States Military intelligence officials have identified the F-91W as a watch that terrorists use when constructing time bombs.[8][9][10][11][12]

This association was highlighted in the Denbeaux study, and may have been used in some cases at the Guantanamo Bay detention camp.[13] 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.[14]


Model Name Description
F-91-WC Neon Colors: Orange, Blue, Green, Pink, Beige and Yellow [15]
F-91-WM Metallic colored cases.
F-94W Different icon arrangement in the display.
W59 Waterproof up to 50m.
A158W, A159W, A163W, A164W Stainless steel band.
A159WGA-1 Stainless steel band, gold color.
F-105W, A168W, A168WG-9 Equipped with electroluminescent backlighting instead of LED in black resin, silver or gold.
LA680 Ladies' version.


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 is a "modest masterpiece" says design critic Stephen Bayley.[2] Counterfeits generally have a slightly lower plastic build quality, the LCD viewing angles are not as wide, and they do not keep as accurate time as the original watches from Casio.


  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 Winterman, Denise (2011-04-26). "The strangely ubiquitous Casio F-91W". BBC News. Retrieved 2017-12-21. 
  3. ^ a b c "F91W-1 Classic Timepiece". Casio. Retrieved April 27, 2011. 
  4. ^ "F-91W-1XY". Casio. Retrieved May 22, 2013. 
  5. ^ "Manual" (PDF). Casio. Retrieved November 26, 2014. 
  6. ^
  7. ^ James Ball (April 25, 2011). "Guantánamo Bay files: Casio wristwatch 'the sign of al-Qaida'". The Guardian. London. 
  8. ^ "USA v. al Qahtani" (PDF). US Department of Defense. November 7, 2005. Retrieved February 27, 2007. 
  9. ^ "Combatant status review board for Mohammed Ahmad Said el Edah". The New York Times. October 6, 2004. 
  10. ^ "Why Am I in Cuba?". Mother Jones. July 12, 2006. 
  11. ^ "Guantanamo Captives Jailed Because Of Digital Watches". WCVB. March 10, 2006. Archived from the original on February 22, 2007. 
  12. ^ "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. 
  13. ^ "Empty Evidence". National Journal. February 3, 2006. Archived from the original on July 4, 2008. 
  14. ^ R. Jeffrey Smith (July 21, 1996). "New Devices May Foil Airline Security". Washington Post. pp. A01. Retrieved March 14, 2008. 
  15. ^ "Collection of Casio F-91WC Variations". 

External links[edit]