Casio F-91W

Why it’s here

The F-91W is the reference point for this entire site. Introduced in June 1989 and never discontinued, never meaningfully redesigned, it is the most-produced watch in history — roughly 100 million units by 2026, at a pace of about three million a year, every year, for three and a half decades. When a $20 product stays in production for 36 years unchanged, that isn’t marketing. That’s natural selection.

The story

The F-91W was designer Ryūsuke Moriai’s first project at Casio — a cost-reduced successor to the F-87W, drawn with one constraint: nothing on the watch that didn’t earn its place. The result was a 21-gram resin slab with a sealed quartz module, three buttons, and a parts count low enough to mass-produce essentially forever.

That’s exactly what Casio did. The watch survived the G-Shock revolution, the smartwatch era, and its own counterfeiting industry without a redesign. It has been worn by presidents, montañeros, and approximately every backpacker on earth. The design brief was “cheap and correct”; thirty-six years later it reads as a philosophy statement.

Durability report

There is very little to break. A sealed quartz module accurate to about ±30 seconds a month, a resin case with no moving external parts except three buttons, and an LCD with no backlight tube to burn out (the famous green microlight is a simple LED). Failure modes, in order of likelihood: the strap (replaceable, ~$5), the battery (CR2016, ~$3), and the case scratching into a matte fog after years of desk diving — cosmetic, not functional.

Owner reports of 20-year examples on original everything-but-the-battery are routine; one decade-long ownership report documents the pattern precisely — straps tear or snap long before batteries die, and nothing else fails at all. The watch has survived deployments, dishwashers, and childhoods. Its longevity track record isn’t merely good; it is the benchmark other products in this category get measured against.

Repair & parts reality

The back is held on by four Phillips screws — no case press, no special tools. A battery swap takes five minutes and a steady hand. Straps, batteries, and even complete replacement 593 modules are commodity items available everywhere watches are sold, and will remain so as long as Casio keeps stamping them out, which appears to be forever.

The honest caveat: beyond battery and strap, nobody repairs an F-91W, because a new one costs less than any repair. It scores well on parts and process, but it is ultimately a replaceable object rather than an heirloom-serviceable one — the only thing separating it from a perfect repairability score.

The long-term math

Twenty dollars. Ten years per battery cycle at a few dollars each. Call it about $2.60 a year to always know the time, with an alarm and a stopwatch thrown in. Even mainstream watch press concedes the uncomfortable arithmetic: on accuracy and durability per dollar, this $20 resin rectangle embarrasses watches costing five figures. There is no product on this site — possibly no product anywhere — with better cost-to-lifetime math. This is the score every other watch has to justify itself against.