G-SHOCK GA-2100-1AER

The Casio G-SHOCK GA-2100-1A is the watch that started it all. Released in 2019 alongside the all-black “stealth” -1A1 and the all-red -4A, this is the original launch reference of the GA-2100 series — the model that became the most influential modern G-SHOCK in years and earned the entire family its enthusiast nickname, the “CasiOak.” Where the -1A1 hides everything in matte tones and the -4A pushes the colour up to maximum, the -1A is the readable, balanced, almost dressy version: a clean black case and resin band, white analog hour and minute hands, light grey applied indices on a smooth black dial, and a small digital LCD reading dark on light. It’s the variant that does the most to sell what the GA-2100 actually is — a slim, minimalist, properly tough G-SHOCK that wears under a shirt cuff.

The case shape is the now-iconic octagonal silhouette, drawn — Casio’s official line — from the very first G-SHOCK, the DW-5000C of 1983, which itself had an eight-sided bezel. The dimensions are 48.5 mm lug-to-lug, 45.4 mm wide and just 11.8 mm thick, and weight is a remarkable 51 grams. That low profile is possible because of Casio’s Carbon Core Guard structure: the case is moulded from a fine resin reinforced with carbon fibre, rigid enough to function as a structural element in its own right rather than just a cosmetic shell. The two-layer back cover adds another layer of impact resistance, and the shock-proof button structure means the buttons sit cleanly along the case sides without the chunky external guards that defined earlier G-SHOCKs. The result is a watch with full G-SHOCK toughness — shock resistance and 200 m water resistance — that genuinely disappears on the wrist after a few minutes.

The dial is the cleanest in the launch trio. The face has a three-dimensional layered construction with a small day sub-dial at 9 o’clock and the small digital LCD in the lower right quadrant, and the layout has been deliberately stripped back: no mode-indicator clutter, no busy printed text, just the indices and the hands. The minute and hour hands carry a Neobrite phosphorescent treatment so the watch stays legible after dark, and the hand shift feature moves the hour and minute hands clear of the digital display when needed so the small LCD stays readable. A double LED Super Illuminator lights both the analog face and the digital display.

Inside is Casio’s module 5611 — Japan-made — driving the standard GA-2100 feature set: a 1/100-second stopwatch with a 24-hour capacity (elapsed, split, 1st–2nd place modes), countdown timer up to 24 hours, five daily alarms with hourly time signal, full auto-calendar to 2099, world time across 31 time zones (48 cities + UTC) with daylight saving on/off, 12/24-hour switching, and a button operation tone on/off function. Battery life is approximately three years on twin SR726W cells. The GA-2100-1A is, in short, the original CasiOak in its most legible, most versatile form — the one that started a family that now stretches to dozens of variants and reshaped how the broader market sees G-SHOCK as a category.

Basic Information

Reference: GA-2100-1A
Collection: G-SHOCK — 2100 Series (“CasiOak”)
Case size (L × W × H): 48.5 × 45.4 × 11.8 mm
Weight: 51 g
Case and bezel material: Carbon-fibre-reinforced resin (Carbon Core Guard structure)
Band: Black resin
Construction: Shock-resistant; Carbon Core Guard; two-layer back cover
Water resistance: 20 bar (200 m)
Power supply and battery life: Approx. 3 years on SR726W × 2

Exterior

Glass: Mineral Glass
Dial: Three-dimensional layered black face; light grey applied indices; white analog hour and minute hands
Hands: Neobrite phosphorescent treatment on hour and minute hands

Watch Features

World time: 31 time zones (48 cities + UTC), daylight saving on/off
Stopwatch: 1/100-second; measuring capacity 23:59’59 (24 hours); modes: elapsed time, split time, 1st–2nd place times
Countdown timer: Up to 24 hours (1-second increments)
Alarms: 5 daily alarms; hourly time signal
Calendar: Full auto-calendar (pre-programmed to 2099)
LED light: Double LED — Super Illuminator (LED for face and digital display); afterglow
Other: Hand shift feature (hands move clear of the digital display); button operation tone on/off; 12/24-hour format
Accuracy: ±15 seconds per month
Regular timekeeping: Analog (3 hands: hour, minute, second) + digital; day sub-dial; date display
Module: 5611