The Casio G-SHOCK G-STEEL GST-B400-1A combines an analog-digital display, solar charging and Bluetooth phone connectivity in a 12.9 mm-thick case. Casio lists it at 46.6 × 49.6 × 12.9 mm and 80 g, with 200-meter water resistance. Those are manufacturer specifications—not independent test results. The available official information supports a useful feature overview, but not a firsthand assessment of wrist comfort, display readability, battery performance or current US availability.
What is the GST-B400-1A?
It is a G-SHOCK G-STEEL GST-B400 Series watch. Casio’s US product page styles the reference as GSTB400-1A, while other official pages use GST-B400-1A; both refer to the title model. When shopping, check that a listing matches the exact reference and the pictured band and finish. Other GST-B400 variants exist and should not be assumed to be identical.
How big is the GST-B400-1A?
Casio lists the case dimensions as 46.6 × 49.6 × 12.9 mm (length × width × height) and the weight as 80 g. Its case and bezel use resin and stainless steel, the band is resin, and the glass is mineral. These figures describe the watch on paper; they do not establish how it will fit or feel on a particular wrist.
Casio describes the case construction as using carbon fiber-reinforced resin to help provide shock resistance while reducing weight. The company calls the 12.9 mm profile the slimmest in the G-STEEL line on its product page. Its series page described it as the “slimmest G-STEEL yet,” based on a Casio survey as of February 2021. That is a dated manufacturer comparison, not a current independent ranking.
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- Rugged & Sporty Design – Matte black finish with bold digital display, built for tactical, military, and extreme sports enthusiasts.
- High-Brightness LED Backlight – Features Super Illuminator LED with a light guide plate, ensuring exceptional visibility in low-light conditions.
- 200M Water Resistance – Engineered for swimming, snorkeling, and outdoor adventures, this shock-resistant watch is ready for action.
- Extended 7-Year Battery Life – Powered by a CR2025 battery, offering years of reliable performance without frequent replacements.
- Shock-Resistant Structure – Built to withstand extreme conditions, making it ideal for sports, military, and rugged outdoor use.
Is the GST-B400-1A solar powered?
Yes. It uses Casio Tough Solar and a rechargeable battery. Casio estimates approximately seven months of operation after a full charge under normal use with no further light exposure. Its separate estimate of approximately 18 months applies after a full charge when the watch is kept in total darkness with power saving enabled. These are conditional manufacturer estimates, not independently verified battery-life results.
Can I connect the GST-B400-1A to my phone?
Yes. The watch supports Bluetooth Mobile Link and pairs with the CASIO WATCHES app. Casio lists automatic time adjustment, easy watch setting, app-based world time for approximately 300 cities, Time & Place, reminders and phone finder. The app’s approximately 300-city world-time feature is distinct from the watch’s own world-time function, which covers 38 cities plus UTC.
Rank #2
- Shock-Resistant Durability: Built to withstand the toughest conditions, this G-Shock watch features a rugged resin case and bezel, perfect for outdoor adventures and extreme sports.
- 200M Water Resistance: Designed for water enthusiasts, it’s suitable for scuba diving, swimming, and snorkeling, providing reliable protection up to 200 meters.
- Electro-Luminescent Backlight: The blue-green EL backlight with afterglow ensures clear visibility of the time, day, and date, even in complete darkness.
- Precision Stopwatch & Countdown Timer: Measure elapsed and split times with the 1/100-second stopwatch, or set up a countdown timer with up to a 24-hour range, ideal for timing workouts or daily tasks.
- Multi-Alarm with Flash Alert: Stay on schedule with a customizable alarm, hourly chime, and flash alert that combines light and sound for added convenience.
Casio’s US product page includes support links, and the official Singapore support page provides GST-B400-1A operation-guide and user-guide materials. Use the model-specific guide for pairing steps and settings. App compatibility with a particular phone or operating-system version was not established here, so check Casio’s current app information before relying on phone functions.
What functions does the watch include?
- World time: 38 cities plus UTC, with home/world time swapping and daylight-saving settings.
- Stopwatch: 1/100-second measurement for the first 60 minutes, then one-second units; elapsed-time and lap/split modes.
- Countdown timer: Settable up to 60 minutes.
- Alarms: Five daily alarms.
- Lighting: Double LED illumination for the analog face and digital display, with selectable 1.5- or 3-second lighting and afterglow.
- Calendar: Full auto-calendar through 2099.
- Hand shift: Moves the hands aside to improve access to the digital display.
What do the durability and accuracy specifications mean?
Casio specifies shock resistance, a Carbon Core Guard structure and 200-meter water resistance. These are the manufacturer’s published specifications; they are not the outcome of independent tests reported here. The water-resistance rating alone does not establish suitability for a particular diving activity or set of conditions.
Rank #3
- Super Matte Finish, Shock Resistant, 200M water resistance, LED light (Super illuminator), World time, Stopwatch, Countdown timer, 5 daily alarms, Hourly time signal, Hand shift feature
- Full auto-calendar, 12/24-hour format, Analog: 2 hands (hour, minute (hand moves every 20 seconds)), Digital: Hour, minute, second, pm, month, date, day, Approx. battery life: 5 years on CR2016
- Quartz Movement
- Case Diameter: 52.8mm
- Water resistant 200m (660ft): in general, suitable for professional marine activity and serious surface water sports, but not diving
Casio lists accuracy of ±15 seconds per month with Mobile Link disabled. The specification is not an independent timekeeping measurement, and the stated accuracy condition matters: it applies when the phone link is off.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Is the GST-B400-1A worth buying?
It is a plausible choice if you want a G-SHOCK with an analog-digital layout, Tough Solar charging and phone-linked time features, and you are comfortable with the listed 46.6 × 49.6 mm dimensions and resin band. Its specification sheet also documents a 200-meter water-resistance rating, stopwatch, timer, alarms and dual LED lighting.
Rank #4
- Band Size: Mens-Standard
- Shock Resistant
- Auto LED Light with Afterglow
- Magnetic Resistant
The available evidence does not establish how easy the displays are to read in everyday use, how the watch wears, how it performs in real-world charging, or how its value compares with competing models. Those questions require hands-on evaluation and current, like-for-like pricing. Casio’s US page did not provide an exposed current price or stock status in the accessed product-page text; check a current listing for the exact model before purchasing. Regional pricing on Casio’s European page should not be treated as US pricing.
If considering a replacement resin band, verify exact GST-B400-1A compatibility and the correct part number before ordering. A band described only as suitable for the broader GST-B400 family may not be enough to establish fit.
Quick Recap
Best Value
- Sport Type: Scuba Diving
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
