Casio G-Shock GA-2100 in-depth: Movement, Finishes and Price 2026

Review of the Casio G-Shock GA-2100

We’ve reviewed the Casio G-Shock GA-2100 based on what really matters: how it performs in real-world use, what compromises the manufacturer has made, and whether its price holds up against direct alternatives such as the Invicta Pro Diver 8926OB. No list of specifications copied from the product sheet: just the features that make a difference to your experience.

Quartz, automatic or manual: which to choose and why

The quartz It is the most accurate and the most practical: it diverts around ±15 seconds per month, it needs nothing more than a new battery every 2–4 years and is more shock-resistant. The automatic It is powered by a wrist movement and deflects between -10 and +30 seconds a day (a COSC-certified calibre has a rate of between -4 and +6), but it is the one with a mechanical soul, a sliding seconds hand and long-term value. The manual It needs to be wound every day and is usually more delicate, which is why it’s used in dress watches. If you’re looking for accuracy, go for quartz; if you’re looking for fine watchmaking, go for an automatic.

Power reserve – and what happens if you don’t have one

Power reserve is the length of time an automatic watch continues to run after you stop moving it: typically, this is 38–41 hours in traditional calibres, which means that if you take it off on Friday night, it will have stopped by Monday. Modern movements have extended that figure: the Powermatic 80 lasts up to 80 hours and some current calibres at 70. For several automatic watches, a rotary box It keeps them running, although this isn’t essential: leaving it switched off won’t damage the mechanism; it just means you’ll have to set the time again. What is advisable, however, is a service every 5–8 years.

The gauges that are actually inside

Almost all automatic cars costing less than €500 are fitted with one of these engines: Seiko NH35/4R35 (reliable, hackable, 41-hour power reserve, the standard for affordable watches), Miyota 8215 (cheap, second hand with a slight wobble) or Miyota 9015 (finer and at 28,800 alternations). Moving up, we find the ETA 2824-2 and its equivalent Sellita SW200, and the Powermatic 80 by Tissot, which stands out for its 80-hour power reserve. Knowing the calibre tells you more about the watch than the brand: two watches with very different price tags may share the same movement.

Accuracy: how to tell if your automatic gearbox is working properly

An automatic watch is not a precision timepiece, and you shouldn’t judge it as such. To measure it properly, set it to the same time for seven days in a row treat it as normal and calculate the average daily deviation: within -10 / +30 seconds a day This is within the factory specification for an uncertified calibre. If it falls outside this range, it is almost always corrected by means of a regulation at a watchmaker’s (€30–60), not as part of a repair. A useful tip: the position in which you leave it overnight affects the result, so if it’s running fast, you can leave it face up, and if it’s running slow, with the crown facing upwards.

• Quality of materials and finishes
• Recent verified reviews (including 3–4-star reviews)
• Warranty and returns policy
• Current price compared with its historical average
• Seller's reputation
• Make sure the key features suit your needs (day-to-day use)

Water resistance: what the ATM rating really means

The figure on the box does not represent the actual working depth; it is a static laboratory pressure reading. In practical terms: 3 ATM / 30 m It can withstand rain and splashes, but nothing more; 5 ATM allows you to wash your hands but not to have a shower (the hot water exceeds that pressure and the heat causes the seals to expand); 10 ATM it’s suitable for swimming; and 20 ATM / 200 m That’s the minimum for diving. If you want a proper diving watch, look for the certification ISO 6425, which can be identified by the word printed on the dial DIVER'S alongside depth.

Criterion	Casio G-Shock GA-2100Our choice	Invicta Pro Diver 8926OB
Ideal for	Balanced and general use	Day-to-Day Life
Strength	Value for money	Specific features
Reviews	Very positive and numerous	Positive
Approximate price	from around €240	from around €150
Recommendation	Our choice	Best alternative
View on Amazon	See price →	See price →

Straps and bracelets: the most cost-effective change

Changing the strap transforms a watch for €20–40 and is the best value-for-money upgrade there is. Measure the width between antlers (18, 20 and 22 mm are the most common) and choose according to your needs: steel for everyday use, skin for washing (never with water), rubber or FKM for sport and summer, and NATO made of nylon for a military look and quick changes. Look for pins made of quick release (quick-release) so that no tools are required. With steel bracelets, the detail that sets the good ones apart from the mediocre ones is the micro-adjustable fastening, which allows you to fine-tune the fit when your wrist swells in the heat.

Final verdict

The Casio G-Shock GA-2100 is a recommended purchase if it matches what you’ve read above, and a disappointment if you were expecting a different category of product. Compared to the Invicta Pro Diver 8926OB, it comes out on top in terms of overall balance but falls short on specific features. Price and availability on Amazon change frequently, so check both before buying.

Frequently Asked Questions

Do sundials need maintenance?

Virtually none. The cell lasts between 10 and 20 years and the capacitor can be replaced; under normal lighting conditions, even artificial light, they remain charged and retain a reserve of power for months in the dark.

How much does it cost to change a watch battery?

Between €8 and €20 at a watchmaker’s, depending on whether the case needs to be unscrewed and the water resistance retested. With a water-resistant watch, it’s worth paying for the pressure test: changing the battery without replacing the gasket is the most common reason for moisture to get in months later.

What size watch suits me best?

Focus on the lug-to-lug measurement rather than the diameter: under 48 mm for wrists measuring 16–17 cm, up to 52 mm if your wrist is over 18 cm. In terms of diameter, 39–41 mm is the most versatile range, whilst 36–38 mm is the classic choice for a slender wrist or a suit.

Do sundials need maintenance?

Virtually none. The cell lasts between 10 and 20 years and the capacitor can be replaced; under normal lighting conditions, even artificial light, they remain charged and retain a reserve of power for months in the dark.

How much does it cost to change a watch battery?

Between €8 and €20 at a watchmaker’s, depending on whether the case needs to be unscrewed and the water resistance retested. With a water-resistant watch, it’s worth paying for the pressure test: changing the battery without replacing the gasket is the most common reason for moisture to get in months later.

Should I go for the Casio G-Shock GA-2100 or the Invicta Pro Diver 8926OB?

The Casio G-Shock GA-2100 is the most well-rounded option and our default recommendation. The Invicta Pro Diver 8926OB makes sense if your priority is everyday use. As a budget guide, the useful price range starts at around €150, and the best value is usually found at around €240.

By the Watch Guide Team · reviewed by

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