Casio F-91W

The F-91W is a rectangular digital watch in black resin with a stainless steel back, a 21 gram all-up weight and a display showing hours, minutes, seconds, day and date at the same time. Casio introduced it in 1989 and has made it continuously since, in numbers that run into the tens of millions.

It costs less than a restaurant meal in most countries, which invites a certain tone of writing about it. That tone is misplaced. The F-91W is a competently engineered instrument that does the thing a watch exists to do, to a standard most mechanical watches cannot reach, and it does it for years without attention.

The specification

What module 593 does

Module 593 is the electronics package: a quartz oscillator, a divider circuit, a display driver, a calendar and a small set of functions. It is stamped on the caseback, which is a useful habit of Casio's, because the module number rather than the model name is what identifies the manual and the settings procedure.

The function set is deliberately small. One daily alarm, an hourly beep, a stopwatch that resolves to a hundredth of a second and counts to just under an hour, and a calendar that is pre-programmed rather than perpetual. February is fixed at 28 days, so in a leap year you correct the date once by hand. Everything is driven from three buttons, and the setting sequence has not changed since 1989.

What is not there matters as much. There is no world time, no countdown timer, no illumination panel, no connectivity and no sensor. Each of those would draw current, and current is what the seven year battery figure is buying.

Accuracy in practice

Casio publishes ±30 seconds per month. That is a worst case across the permitted temperature range, and it works out to roughly one second a day.

Put that against the mechanical scale. A Swiss chronometer certified under the COSC procedure must average between minus 4 and plus 6 seconds a day, and a good uncertified automatic will typically run somewhere in a ten to twenty second a day band. The F-91W's published tolerance sits inside chronometer territory, and individual examples often do better than the specification because the tolerance has to cover a range of temperatures the watch will rarely see.

This is not a trick, and it is not a reason to think less of mechanical watches. It is the ordinary consequence of regulating a timepiece with a crystal that vibrates tens of thousands of times a second instead of a wheel that swings a few times a second. Our page on how accurate a quartz watch is covers the mechanism, and quartz against automatic covers what you give up in exchange. If you want to measure your own, the accuracy calculator converts a drift over a week into seconds per day.

The battery, and the cost of changing it

The CR2016 is rated at about seven years. That is an unusually long interval for a watch with an alarm and a light, and it is the direct result of the small function set and the low-power display.

Changing it is where the economics get strange. The steel back is held on by four screws, so it is a straightforward job for anyone with the right screwdriver, a clean surface and the discipline not to touch the circuit board. Paid for at a shop, a battery change plus a gasket can cost as much as the watch, or more. Many owners simply buy another watch, which is rational and slightly uncomfortable to write down.

How it wears

At roughly 38 mm long, 35 mm wide and 8.5 mm thick, the F-91W is smaller than almost any current mechanical watch and it slides under a shirt cuff without catching. The 21 gram weight is the more important figure. It is light enough that people forget they are wearing it, which is why it turns up on wrists that would never tolerate a 150 gram steel bracelet.

The band is soft resin with a taper, and it fits genuinely small wrists, including children's, as well as large ones. There is no size at which it looks correct and no size at which it looks wrong, because it makes no attempt at proportion. If you want to compare it to something with a conventional case, the watch size calculator and the sizing and fit guide use lug to lug length as the governing measurement, and 38 mm of overall length is very short indeed.

The band is the failure point

Almost every F-91W that gets thrown away is thrown away for the same reason: the resin band split, usually at the narrow section next to the case or at the buckle holes. The module is still running.

The cause is ordinary polymer aging. Polyurethane and similar resins harden as plasticizers migrate out, accelerated by ultraviolet light, heat and skin oils, and the band is thinnest exactly where it flexes most. Two to five years is a common life, and hot climates are at the short end of that.

The fix is cheap and slightly awkward. Casio sells replacement bands as spare parts, and they are held by spring bars, so replacing one is a five minute job with a spring bar tool. The awkwardness is that the replacement band costs a noticeable fraction of a whole new watch, so the rational move is often to buy the watch again. Our straps and bracelets guide covers the general problem of resin aging.

What is wrong with it

The light is poor. A single side-mounted LED illuminates the display unevenly and leaves the far corner dark, and it is the complaint that comes up most among people who have worn one for years.

The window is plastic, so it scuffs. Light marks buff out with a plastic polish, the same trick that works on the acrylic crystals of many vintage watches, but it will never stay unmarked for long. Water resistance is minimal, and the watch should be kept out of showers and pools. The alarm is quiet enough that it will not reliably wake a heavy sleeper. And the display, while legible in daylight, has small day and date fields that older eyes find hard.

None of these are defects. They are the specification, and the price is the reason for the specification.

Why it has lasted since 1989

Four reasons, none of them nostalgia. The tooling and the module design were paid for decades ago, and enormous production volume keeps the unit cost at a level nobody has undercut with anything as reliable. The function set is small enough to give a seven year battery, which means the watch demands attention roughly twice a decade. The case is light enough to be worn by anyone. And the design was never fashionable, so it has never gone out of fashion.

The result is a watch that is bought as a stopgap and kept for a decade. If you are assembling a first collection, it is worth understanding what it does well before you decide how much to spend on a watch or start reading about affordable brands. Casio's wider range is covered on the Casio brand page.

What it costs, and what to buy instead

Indicative US figures, quoted before sales tax and moving with distribution and discounting: the F-91W lists at around $20 and sells on the street for roughly $15 to $30 new, with prices in other countries set by local tax and import duty rather than by converting the dollar figure. There is no meaningful used market and no reason to shop in one. Over ten years, expect one battery change and one or two bands, which together may exceed the purchase price, an inversion the cost of ownership calculator makes visible.

If you need water resistance and impact protection, the G-Shock DW-5600 is the same idea built to survive, at roughly three times the price and 200 meters of rating. If you want an analog Swiss watch with the same low-maintenance logic, the quartz version of the Tissot PRX is a different budget entirely but the same argument about not needing winding. If you want a mechanical watch and know that it will be less accurate and more demanding, start with buying your first mechanical watch.

Buy the F-91W if you want accurate, unobtrusive, unbreakable-in-practice timekeeping and do not care what it signals. Do not buy it as a swimming watch, a night watch or an object you expect to repair.