Casio announced a noticeably thinner version of its affordable metal-clad G-Steel timepiece in May. The new collection consists of three references, and we went hands-on with two of the models.

The new slimmer G-Steel is not just the thinnest wristwatch ever within the line — the entire case has been reduced in size, resulting in one of the most wearable G-Shock timepieces we’ve ever tested. At 41 mm in diameter without the case guards (49 mm including the case guards), and just 12.99 mm thick (Casio states 12.9 mm), the stainless steel, resin, and carbon fiber case is great on paper. But how does it feel on the wrist?

G-Steel GSTB400D-1A

Most importantly, the G-Shock aesthetic is maintained but without the typical bulk. There’s wrist presence, although not like you’re used to. Compared to its predecessor’s 14.5 mm thickness, the approximate 1.5 mm reduction is much appreciated, however, it’s the lug-to-lug measurement of just 46.5 mm that impressed me even more than the reduced height. On my 7″ wrist I found that watch wears more like a Swiss timepiece than a typical Casio, or Seiko sports watch for that matter. I don’t need the watch to be 38 mm in diameter and 10 mm in thickness to wear well (although those with smaller wrists might prefer that), and I tend to gravitate toward watches in the 41 mm range, give or take a millimeter.

G-Steel GSTB400D-1A overhead

Casio sent us two loaners (as denoted by the “S” engraving on the caseback). One was the G-Steel GSTB400-1A (steel case with rubber strap) and the other was the GSTB400D-1A (steel case with steel bracelet), to try out. We have not seen or tested the GSTB400BD1A2 (black PVD steel with matching bracelet), but like the other two, Casio chose a pretty attractive colorway. Clearly, a lot of effort was put into every detail of this new G-Steel collection.

G-Steel GSTB400-1A

With the rubber strap attached, the G-Steel GSTB400-1A weighs in at just 78.6 grams, which considering the metal case and robustness, is rather light. The bezel, top of the case, buttons, screws, and caseback are stainless steel, while the case middle and interior mounts are made using Casio’s Carbon Core Guard technology which blends carbon fiber and resin to create a highly shock-resistant structure that’s also ultra-lightweight. Perhaps best of all, you can see the marbled carbon fiber-resin material when viewing the case from the side and the caseback. The aesthetic is similar to the forged carbon you see on high-end watches costing huge sums, yet is presumably far cheaper to produce thanks to the use of resin as opposed to a purer carbon alloy, which makes us wonder if, or when, we might see this material used more of the outer case. After all, the Carbon Core Guard is already being used within other G-Shock collections, such as the MT-G. And unlike the higher-end forged carbon alloys, Casio’s is far more durable as it’s used not just as an integral part of the case but also to increase shock resistance.

G-Steel GSTB400D-1A profile

Once you add the metal bracelet, the total weight more than doubles, at 160.4 grams, but thanks to excellent ergonomics, and the fact that a watch this size with a metal bracelet typically weighs more, it does not feel heavy at all. That’s again thanks to the Carbon Core Guard structure — which probably shaves 20-40 grams off compared to a similar all-steel watch.

G-Steel GSTB400-1A wristshot

One of the new design changes for the 2021 G-Steel GSTB400 is the omission of a crown. The GSTB300 that was introduced in 2020, still had the digital crown system, but Casio decided to do away with it in favor of a simple tried-and-true four-button system that gives the watch a sleeker look, even with the hinges (case guards) located at 3 and 9 o’clock. A crown, whether mechanical or digital, is always a source of potential weakness on a watch, so to eliminate that while gaining better ergonomics — as has been the case with the majority of G-Shocks since being first introduced in 1983 came — is a logical design choice.

G-Steel GSTB400-1A and Casio AMW320R-1EV

Although there’s no indication that the GSTB400 was designed with inspiration from any specific past models, dimensionally, we could not help but notice how comparable the vintage Casio AMW320R-1EV looks when placed next to the modern GSTB400. Could be a coincidence but it’s probably not considering how many product companies — whether we’re talking about a car or a watch — use past models for inspiration. Notice how the lug-to-lug distance is shorter than the vintage timepiece, although overall the two watches (produced over three decades apart), are rather close in size. The AWM320R-1EV, which we previously reviewed, is thinner, at 11 mm, but it’s also a standard Casio, not a highly durable and shock-resistant G-Shock. Nor does it have automatic solar-powered movement or Bluetooth time sync capabilities. The ability of the G-Steel to power itself and adjust the time automatically (when connected to Bluetooth via the G-Shock app), are the two G-Shock features that modern consumers that we’ve spoken with generally desire across the board. G-Shocks with solar and Bluetooth represent an important niche for Casio because they do not fall into the mechanical watch category, they’re not a standard battery-power quartz, nor are they a smartwatch. This advanced type of G-Shock literally defines its own category.

G-Steel GSTB400D-1A overhead angled view

While it may not be apparent in the photos, the GSTB400D-1A with a steel case and black and gray color scheme on the dial makes the mineral crystal protecting the dial looked almost tinted. The monotone color scheme looks excellent, although that GSTB400-1A, with a rubber strap and red and white highlights on the dial, is attractive as well. There’s also an all-black PVD stainless steel model, GSTB400BD1A2, with a blue-themed dial, that looks equally sharp. All models have a flat mineral crystal (at this price point you’re unlikely to get a sapphire crystal), and the crystal is set about 1-2 mm below the plane of the metal bezel, protecting it from impacts and scratches. It’s important to note that a mineral crystal is easily scratchable compared to sapphire but not when it’s armored like this. A mineral crystal is also less brittle and likely to shatter than a sapphire crystal, so you can beat this watch up by design, and not worry too much about a shattered or scratched crystal.

G-Steel GSTB400D-1A close-up

On the wrist, the near-perfect dimensions, the lightweight feel (thanks to the Carbon Core Guard), and ideal ergonomics are just right. Not to mention, visually, the vertical brushing of the steel bezel and the mixture of polished and brushed surfaces over the different angled and faceted elements, create a watch that’s as attractive and as it is comfortable.

G-Steel GSTB400-1A caseback

Lastly is the dial, which has the depth and three-dimensional aesthetic G-Shocks are known for but it’s been completely streamlined, even compared to the G-Steel GSTB300 that came out last year. The hands are sleeker, yet still highly prominent which enhances readability. And while there’s only lume on the hour and minute hands, the on-demand LED light is super bright so you can literally read the time, at a glance, with ease, in any lighting environment. The subdial arrangement has been rearranged in a way that’s both more pleasing aesthetically and simpler to read. Not to mention, the readouts have less superfluous functions, for instance on the image just above this paragraph, you can see the day readout “FRI” at 6 o’clock, and this can also be toggled to show a second time zone. Perhaps my favorite feature, is that you can use the subdial at 3 o’clock as part of the stopwatch function, or simply as a running seconds readout that means you read the main time from analog hands (hours and minutes), and then the seconds, from the dedicated readout, digitally.

G-Steel GSTB400D-1A wristshot


Like the watch’s hybrid solar and battery power quartz movement, and Bluetooth time sync system, the dial display is also a hybrid and even a rugged case mixes numerous materials. G-Shock has really stepped up with the latest G-Steel line, taking the best parts from numerous different technologies and production processes — with the ultimate beneficiary being the consumer. This is my favorite G-Shock, resin or metal, introduced in the past two years, if not longer.

Retail is $320 for the G-Steel GSTB400-1A (with a black resin strap), $400 for the GSTB400D-1A (on a steel bracelet), and $500 for the GSTB400BD1A2 (in black PVD steel with matching bracelet).

Learn more at G-Shock.



G-Steel GSTB400

Reference: GSTB400-1A (resin strap)/GSTB400D-1A (steel bracelet)

Year: 2021

Total Weight: 78.6 grams (resin strap)/160.4 grams (steel bracelet)

Case Diameter: 41 mm (49 mm including the hinges)

Case Thickness: 12.99 mm

Lug-to-Lug: 46.5 mm lug-to-lug

Lug Width: 27 mm at the lugs to 19.5 mm at the buckle (resin strap)/27 mm at the lugs to 19.75 mm mm at the clasp (steel bracelet)

Crown Diameter: No crown

Glass: Mineral crystal (armored)

Posted by:Jason Pitsch

Jason is the founder of Professional Watches and specializes in writing, product photography, and digital marketing.