Casio F-91W vs Seiko Prospex "Turtle"
Casio's timepiece meets Seiko's diver
Quick Verdict
The Casio F-91W (Under $50) offers Casio Digital Quartz (Battery) precision in the tier below the Seiko Prospex "Turtle" ($500–$1,000). The F-91W measures 35mm and is rated 30m (Splash Resistant). The Prospex "Turtle" counters with Made in Japan craftsmanship and 200m (20 ATM) Diver water resistance. Both are exceptional watches for their respective tiers.
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Specifications Compared
| Feature | Casio F-91W | Seiko Prospex "Turtle" |
|---|---|---|
| Price | Under $50 | $500–$1,000 |
| Case Size | 35mm | 45mm |
| Movement | Quartz Module 593 | Automatic Caliber 4R36 |
| Caliber Type | Casio Digital Quartz (Battery) | In-House Automatic Manufacture |
| Power Reserve | 7-Year Battery | 41 Hours |
| Water Resistance | 30m (Splash Resistant) | 200m (20 ATM) Diver |
| Crystal | Acrylic | Sapphire Crystal |
| Case Material | Resin | Stainless Steel |
| Origin | Casio (Japan) | Made in Japan |
Category-by-Category Analysis
Design & Aesthetics
Casio's design language vs Seiko's approach
The Casio F-91W features a Resin case at 35mm with Acrylic protection. The Seiko Prospex "Turtle" offers Stainless Steel construction at 45mm with Sapphire Crystal. Both represent their brand's distinct design philosophy and heritage.
Movement & Performance
Quartz Module 593 vs Automatic Caliber 4R36
The F-91W runs the Quartz Module 593 (Casio Digital Quartz (Battery), 7-Year Battery power reserve) at standard frequency. The Prospex "Turtle" uses the Automatic Caliber 4R36 (In-House Automatic Manufacture, 41 Hours reserve) at 21,600 vph (3 Hz). The Prospex "Turtle" offers a longer power reserve.
Wearability & Fit
35mm vs 45mm — different wrist presence
At 35mm, the F-91W wears more compactly on smaller wrists compared to the Prospex "Turtle" at 45mm. Water resistance differs: 30m (Splash Resistant) vs 200m (20 ATM) Diver.
Value & Cost of Ownership
Casio sits in the lower tier
The Casio F-91W sits at Under $50 and the Seiko Prospex "Turtle" at $500–$1,000. On the specs that outlast any price: the F-91W uses Acrylic with 7-Year Battery, against Sapphire Crystal and 41 Hours on the Prospex "Turtle". Service intervals are typically 5-7 years for both.
Water Resistance
30m (Splash Resistant) vs 200m (20 ATM) Diver
The F-91W is rated to 30m (Splash Resistant) while the Prospex "Turtle" handles 200m (20 ATM) Diver. The Prospex "Turtle" offers superior water resistance.
Our Verdict
Casio
F-91W
Choose the Casio F-91W if you prioritize Casio Digital Quartz (Battery) engineering, Casio (Japan) craftsmanship, and 30m (Splash Resistant) water resistance. At Under $50, it delivers Quartz Module 593 with 7-Year Battery power reserve.
Check Price on AmazonSeiko
Prospex "Turtle"
Choose the Seiko Prospex "Turtle" if you value In-House Automatic Manufacture technology, Made in Japan heritage, and 45mm proportions. At $500–$1,000, the Automatic Caliber 4R36 with 41 Hours power reserve makes it a compelling choice.
Check Price on AmazonThe Bottom Line
The Casio F-91W and Seiko Prospex "Turtle" represent two distinct approaches to fine watchmaking — the Casio bringing Casio (Japan) tradition while Seiko delivers Made in Japan engineering.
Frequently Asked Questions
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How This Page Was Made
Our comparisons are generated from a hand-maintained specification database and reviewed under our editorial standards. We use AI tools to assemble pages from that data. We don’t invent specs: every number traces back to the database, and when we correct an entry, the fix is applied site-wide. As an Amazon Associate, The Horologist earns from qualifying purchases.