The Casio F-91W-1 is a compact resin digital watch with a daily alarm, hourly time signal, LED light, calendar and a 1/100-second stopwatch. Casio specifies a CR2016 battery lasting approximately seven years and accuracy of ±30 seconds per month. The important qualification is water resistance: the official page says only “Water Resistant,” so do not assume the watch is suitable for swimming or diving without checking the manual for your exact regional version.
What is the Casio F-91W-1?
The F-91W-1 is a small, lightweight digital watch built around straightforward timekeeping rather than connected or smart features. It shows the hour, minutes and seconds, along with PM status, date and day. Its functions are controlled from the physical buttons on the case, so there is no charging cable, app or touchscreen to maintain.
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Casio’s official product listing describes the model as the F-91W-1. Regional packaging, manuals and availability can differ, so check the model marking and the support information for the watch you are buying rather than assuming every F-91W variant has identical documentation.
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| Specification | Casio’s listed value |
|---|---|
| Case dimensions | 38.2 × 35.2 × 8.5 mm |
| Weight | 21 g |
| Case and band material | Resin |
| Glass | Resin glass |
| Compatible band size | 145–200 mm |
| Water-resistance wording on cited official page | Water Resistant |
| Accuracy specification | ±30 seconds per month |
| Battery | CR2016 |
| Approximate battery life | Seven years |
These measurements make the watch notably compact: the 8.5 mm thickness and 21 g weight are easy to accommodate under a shirt cuff or on a smaller wrist. Resin keeps the case and strap light and flexible, while resin glass is the transparent cover over the display. Casio lists the compatible band range as 145–200 mm; measure your wrist before ordering a replacement strap, especially if you are near either end of that range.
#1 Best Overall
- Micro Light, Hourly Time Signal, Auto Calendar
- 12/24 Hour Formats
- Dial Code: Digital
- Water Resistant
- Daily Alarm
The complete specification is on Casio’s F-91W-1 product page. The linked page is an India regional listing, so local packaging, retailer terms and stock may differ elsewhere.
How long does the F-91W battery last?
Casio gives the F-91W an approximate seven-year battery life using one CR2016 coin cell. “Approximate” is important: it is a manufacturer estimate, not a guarantee that every watch will run for exactly seven years. The actual interval varies with use, including how often the LED light, alarm and stopwatch are operated.
A fading display, an intermittent light, a watch that loses its display after an alarm sounds, or a complete loss of timekeeping can indicate a depleted cell. Those symptoms can also result from another fault, so do not treat them as proof that the battery alone is responsible.
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The listed cell is a CR2016. Buy a replacement identified as CR2016 and verify the marking and polarity against your watch module or manual before fitting it. Coin cells that look similar are not automatically interchangeable; the chemistry, dimensions and electrical characteristics must match the specification.
Should you replace the battery yourself?
Opening a digital watch can disturb the case seal, retaining clips or module contacts. If you are not familiar with small watch modules, use a qualified watch technician and ask for the case-back seal to be checked during service. If you do work on it yourself, follow the precautions in Casio’s module documentation rather than relying on a generic coin-cell tutorial.
How accurate is the Casio F-91W?
Casio rates the watch at ±30 seconds per month. That is a specification for expected timekeeping tolerance, not a promise that every month will show the same gain or loss. Temperature, battery condition and the individual module can affect the result.
At the stated limit, a watch could gain or lose roughly one minute over two months, but the specification should be judged over a longer interval rather than by a single day’s difference. If you need tighter timing, compare the displayed time with a trusted reference, record the difference over several weeks and adjust the watch using the official setting procedure. A small, consistent offset within the published tolerance is not necessarily a defect.
Rank #2
- AN ICON SINCE 1989 — THE WORLD'S MOST POPULAR DIGITAL WATCH — Introduced in June 1989 as a successor to the F-87W, the F-91W is popular for its low price, long battery life, and iconic design. As of 2026, production of the watch has reached 100 million units, making it the most sold watch in the world. A timeless design, unchanged since launch, that has earned a global cult following across generations.
- FEATHERLIGHT & SLIM — Urethane resin is employed for the case and band, and the module features a reduced number of components — every element of construction was reconsidered to secure high quality at an affordable price, resulting in a global hit watch that offers both affordability and long-lasting reliability. The case measures 38.2 × 35.2 × 8.5mm and weighs just 21 grams — barely noticeable on the wrist.
- 1/100-SECOND STOPWATCH WITH SPLIT TIME — The 1/100-second stopwatch covers elapsed time, split time, and 1st-2nd place timing up to 59'59.99" — precise enough for sports, training, cooking, and any timed activity. Handy features for day-to-day life also include a daily alarm, water resistance, and LED light functions.
- DAILY ALARM | HOURLY CHIME | AUTO-CALENDAR | LED LIGHT — Module 593 delivers functions for a daily alarm, chronograph/stopwatch, annual calendar, and hourly chime, with selectable 12 or 24-hour format. The auto-calendar and LED backlight round out a complete, practical feature set in a package that fits any wrist from 145 to 200mm.
- 7-YEAR BATTERY | MODULE 593 | 18MM LUG WIDTH — Powered by a CR2016 battery with an approximate 7-year life — often lasting 10 or more years in practice. The F-91W has an 18mm lug width, meaning standard 18mm NATO straps fit directly without any modification — making it one of the most customizable and mod-friendly watches available at any price point.
Can you swim or shower with an F-91W?
The cited official Casio page labels the watch “Water Resistant” but does not state a metres or bar rating. That wording alone does not establish suitability for swimming, diving or prolonged immersion. Everyday splashes should not automatically be rewritten as a swimming guarantee.
For a decision about showering, swimming or other water exposure, check the manual and markings supplied with your exact regional watch. Casio’s guidance can vary by model documentation and region. If the manual does not clearly authorize the activity, the conservative choice is to remove the watch before immersion.
Water resistance can also be compromised after opening the case for a battery change or after damage to the case, crystal or buttons. A watch that previously tolerated splashes should not be assumed to retain the same protection after service unless the seal has been inspected and the case has been properly closed.
What stopwatch functions does the F-91W have?
The stopwatch measures to 1/100 of a second and has a maximum measuring capacity of 59 minutes, 59.99 seconds. Casio lists three operating modes:
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- Elapsed time: measures one continuous interval from start to stop.
- Split time: displays an intermediate reading while the underlying timing continues.
- First- and second-place timing: records the finishing times of two competitors.
The 59’59.99” capacity means the counter reaches one hour minus one hundredth of a second; it is not a multi-hour stopwatch. For longer events, record the result before the capacity is reached or use a timer designed for that duration.
The display’s hundredths resolution is useful for short intervals, but the watch’s overall timekeeping accuracy remains the separately stated ±30 seconds per month. Stopwatch display resolution should not be confused with laboratory-grade timing accuracy.
Does it have an alarm, hourly signal and light?
Daily alarm
The F-91W includes a daily alarm. It can be set to sound at a chosen time each day and is separate from the normal time display. Use the module instructions to select the alarm mode, set the time and enable or disable the alarm indicator.
Rank #3
- Universal Compatibility: Perfectly fits Casio F-105W-1ASV/F-105W-1ASVY/F-105W-1ASW/F-105W-1ASWY/F-91W-3W/F-91WG-9W/F-94WA-8W/F-94WA-9W/F-91WG-9V/F-105W-9V 05W-1AV/F-106W-1A and other 18mm lug-width Casio watches. Ideal for both men and women.
- Durable and Comfortable: Made of flexible TPU material, offering sweat resistance, lightweight comfort, and long-lasting durability for daily wear or sports activities.
- Easy Installation：Equipped with a sturdy stainless-steel buckle, the installation and adjustment process is a breeze.Additionally, a free set of matching spring bars and an installation tool are included. Even if you're replacing the strap for the first time, you can handle it easily.
- Sleek and Versatile Design: Classic style complements any outfit, from casual to professional, while maintaining the iconic Casio look.
- Complete Package: Comes with one 18mm TPU strap and three stainless steel spring bars, ensuring a hassle-free replacement experience. If you encounter any problems during or after purchase, please contact lddal after-sales, thank you for your support.
Hourly time signal
An hourly time signal can provide an audible reminder at the top of each hour when enabled. It is independent of the daily alarm, so check the indicators separately if the watch beeps when you do not expect it.
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The watch has an LED light for reading the display in low light. Light use draws power from the same CR2016 cell, which is why heavy use can change the practical battery interval from Casio’s approximate seven-year estimate.
How does the calendar work?
The F-91W has an auto-calendar programmed for February’s 28-day cycle. It displays the date and day and supports a 12- or 24-hour format. Because the calendar is specified for 28 days in February, it does not automatically account for leap years or the varying length of every month in the way a full perpetual calendar would. Check the date manually when February ends in a leap year and after any month whose length differs from the watch’s programmed sequence.
The 12/24-hour choice changes how the hour is displayed; it does not change the alarm or stopwatch functions. PM appears in 12-hour mode when applicable.
Where is the official F-91W manual?
Casio’s operation-guide link directs to the official support portal for module 593: Casio module 593 manual portal. Use that page for the button sequences and precautions specific to this module, including setting the time, configuring the alarm, operating the stopwatch and handling the battery.
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- Read the module 593 guide before pressing buttons while in a setting mode. The guide identifies each button and the order used to move between fields.
- Set the time and date first, selecting 12- or 24-hour display as required.
- Configure the daily alarm and confirm its on-screen indicator; enable the hourly signal separately if you want it.
- Run a short stopwatch test in elapsed-time mode, then review the split and first/second-place procedures in the guide before using them for an event.
- After any battery service, verify the time, date, alarm and light. A module reset or interrupted battery change can leave settings needing re-entry.
These steps deliberately point you to the module guide for the actual button sequence. Button combinations can be easy to misremember, and the official manual is the authoritative source for this module.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Can you replace the F-91W strap?
Yes, a replacement resin strap is a practical ownership part, provided it fits the case and the wearer. Casio lists a compatible band size of 145–200 mm. Confirm the strap’s attachment dimensions and hardware as well as its wrist-length range; a strap advertised only by wrist size may still use the wrong mounting geometry.
Rank #4
- Water Resistant, Micro Light
- Daily Alarm, 1/100 second digital stopwatch
- Hourly Time Signal, Auto Calendar, Accuracy: +/-30 seconds per month
- Battery CR2016, Approx. battery life: 7 years
- Module 593, Size of case / total weight: 37.5 x 33.5 x 9.5mm / 20g
Inspect the spring bars and the case lugs when changing a strap. If a bar is bent, corroded or does not seat fully, the watch can detach even when the new resin band is the correct length. Keep the original strap until the replacement has been installed and checked for secure attachment.
What should you check before buying an F-91W?
- Model: confirm that the listing says Casio F-91W-1 if that is the version you intend to buy.
- Condition: decide whether you are buying new, used or refurbished; case wear and a partially depleted battery affect ownership.
- Battery: a CR2016 is the specified cell, with an approximate seven-year manufacturer estimate.
- Fit: compare your wrist measurement with the published 145–200 mm band range.
- Water use: read the regional manual instead of inferring a swimming rating from “Water Resistant.”
- Documentation: retain access to the module 593 guide for setup and servicing precautions.
The direct watch is the primary purchase. A CR2016 replacement cell and a compatible resin strap are the two most relevant task-specific accessories. Retailer stock, prices, regional warranty terms and marketplace participation change over time, so verify those details at the point of purchase rather than treating one seller’s listing as a global specification.
What are the most common F-91W ownership problems?
The display is blank
Start with the battery: the CR2016 may be depleted or improperly seated. If a fresh, correctly oriented cell does not restore the display, stop repeatedly opening the case and have the module and contacts inspected.
The watch loses time quickly
Compare the measured drift with the ±30-seconds-per-month specification. A short observation can exaggerate the apparent error; log the difference over several weeks. A large or rapidly changing error warrants service rather than repeated manual corrections.
The alarm or hourly signal seems silent
Check the relevant indicator and confirm that the feature is enabled in the module’s alarm or signal mode. The daily alarm and hourly signal are separate settings, so changing one does not automatically change the other.
The light is weak or intermittent
Heavy light use can contribute to battery drain. If the light remains weak after a correct battery replacement, the switch, LED or module may need inspection.
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The watch has been immersed
Because the cited page gives no metres or bar rating, treat unexpected immersion seriously. Dry the exterior, avoid pressing buttons while wet and consult a qualified technician if moisture appears under the crystal or the display behaves abnormally.
Is the F-91W a good fit for your needs?
It suits someone who wants a very light digital watch with dependable everyday functions: basic time and date display, an alarm, hourly signal, backlight and a short-duration stopwatch. Its small case and 21 g weight favor unobtrusive wear, and the seven-year battery estimate reduces routine battery changes.
It is less suitable when you require a published swimming or diving rating, a calendar that automatically handles leap years, a stopwatch capacity beyond 59 minutes 59.99 seconds, or timekeeping tighter than ±30 seconds per month. Those are specification boundaries, not defects, and they should guide your choice before purchase.
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