What size strap does a Casio F-91W take?

An 18mm strap. The F-91W's lug width is 18mm with standard spring bars, and the same 18mm fitting is shared by the A158WA, A168WA, AE1200 (Royale), AE1300, and B640W. Casio's own replacement resin strap for the F-91W is a no-straight-mount part fixed by pins, so for standard 18mm straps you reuse the watch's original spring bars.

Is the Casio Duro MDV106 really 22mm?

Yes. Some retail pages round down to 20mm, but caliper measurements and independent reviews agree the MDV106 uses 22mm lugs with standard spring bars. Buy 22mm.

Can I put a normal strap on a G-Shock?

Generally no. Most G-Shocks use a proprietary integrated resin attachment with no standard lug gap, so an ordinary 16 to 24mm strap has nothing to bolt onto. Some models accept aftermarket adapters, but the strap question for a G-Shock is a parts lookup by reference, not a width lookup.

Casio Watch Strap Size by Model

Casio is a strange brand to buy straps for: the classic digitals are the cheapest, easiest strap swap in watches, the Duro diver is a normal 22mm diver, and the G-Shocks, the line most people own, do not take standard straps at all. This page gives the strap width between the lugs for the Casio models people search for, with the proprietary-attachment warnings where they apply. Every row is confirmed by two independent sources.

The one-line answer: the square digitals (F-91W, A158, A168, AE1200 Royale, B640W) all take 18mm straps on standard spring bars, the Duro MDV106 takes 22mm, the analog Edifice line runs 20 to 22mm, and G-Shocks take no standard strap at all, only model-specific resin bands and adapters.

Casio strap width by model line

Model line	Case	Strap size	Attachment
F-91W (and F-84, F-94, F-105, F-108)	35mm	18mm	Standard spring bars
A158WA / A159 (vintage steel)	33-35mm	18mm	Standard spring bars
A168WA (Illuminator)	37mm	18mm	Standard spring bars
AE1200 / AE1300 ("Royale" and siblings)	42mm	18mm	Standard spring bars
B640W (Vintage Edgy)	35mm	18mm	Standard spring bars
CA53W Databank calculator	38mm	20mm	Standard spring bars
Duro MDV106 (all colorways)	44mm	22mm	Standard spring bars
Edifice EF-527D and sport chronographs	42-45mm	22mm	Standard spring bars
Wave Ceptor LCW-M170TD	40mm	20mm	Standard spring bars
MTP / LTP analog dress line	varies	18 / 20 / 22mm	Standard spring bars, by reference
G-Shock DW5600 (square)	43mm	no standard lug	Proprietary integrated resin
G-Shock GA-2100 (CasiOak)	45.4mm	16mm proprietary	Integrated; adapters allow some 22mm straps
G-Shock GA100 / GA140 line	51-55mm	18 / 20mm proprietary	Proprietary integrated resin

The 18mm square family, and the resin trap

The F-91W's 18mm lug width is confirmed by Casio's official specifications and shared across the whole square-digital family: A158WA, A168WA, AE1200, AE1300, B640W, and the F-84 through F-108 siblings. Any standard 18mm strap fits on the watch's own spring bars, which is why the F-91W has an entire internet subculture devoted to putting it on nylon and mesh. The trap is buying Casio's own replacement strap by width alone: the OEM F-91W resin band (Casio part 71604002) is a 22mm-wide no-straight-mount part fixed with pins, shaped for that case only. Aftermarket standard 18mm straps are the simpler path, reusing the original spring bars.

On the B640W, note the difference between the watch's fitting and its stock bracelet: dealer spec sheets list an 18mm lug width but a 20mm band width, because the stock bracelet's first link overhangs the lugs. An 18mm strap is the correct replacement; a 20mm strap will not fit between the lugs.

The Duro, and the 20mm myth

The MDV106 Duro is listed at 20mm on some retail pages. It is 22mm. Caliper measurements and independent reviews agree: 22mm lugs, standard spring bars, 44mm case, roughly 48.5 to 49mm lug-to-lug. Buy 22mm; it is also the best-stocked width in the strap market, so the Duro is one of the easiest divers to re-strap at any budget. For whether the 44mm case suits your wrist, that is the separate case size by wrist question.

G-Shocks: why there is no width to look up

Most G-Shocks have no conventional lug gap at all: the resin band is integrated into the case with a proprietary slide-and-click or bolted attachment, so a standard strap has nothing to attach to. The widths quoted in retailer tables (a DW5600's band is roughly 24mm at the case, a GA-2100's connector is 16mm) describe the proprietary part, not a standard lug width. Third-party adapters exist for popular references like the GA-2100 and open the door to standard 22mm straps, but that is a per-reference parts question. The Casio model number on the caseback, and the parts lookup built on it, is the only reliable route for G-Shock band replacement.

Finding your own number if the table misses it

Casio prints the module and model number on the caseback, which is what retailer fitment pages index by. If your reference is missing above, measure the inside gap between the lugs: the method is in the measuring guide, and the lug width chart explains what the number means for shopping. The watch database carries the Duro's and CasiOak's case dimensions if you need those too. For the other big Japanese brands, see the Seiko band size chart (watch for Seiko's fat spring bars on divers) and the Citizen band size chart.

Related guides

Once you know the width: watch band materials compared for what to buy, strap length by wrist so it fits around the wrist and not just the lugs, and how to adjust a watch band, whose Casio section covers the collar pins and clasp-hole tricks these bracelets use. Changing the strap itself is a spring bar job: the spring bar tool guide covers the tool, and spring bar sizes the pins. Also here: the Timex band size chart and the watch strap taper chart.

Sources

F-91W 18mm and the family sharing it: Casio's official specifications as cited by CNS Watch Bands' F-91W guide and digital-watch strap guide (A158/A168/AE1200/F-84 through F-108 all 18mm), corroborated by Fuerman's 18mm Casio square-family replacement listing.
F-91W OEM resin strap 22mm no-straight-mount on pushpins: Masters in Time's official-dealer listing for Casio part 71604002, corroborated by Esslinger's factory Casio band listings.
B640W 18mm lugs / 20mm stock band: Auer.lu's dealer spec sheet for the B640WC (lug width 18mm, strap width 20mm), corroborated by Fuerman's and Amazon 18mm Casio square replacement listings that include B640.
AE1200 18mm: Casio's own OEM strap listing, part 10365960, which states lug width 18mm for AE1200WH and AE1300WH variants, corroborated by CNS Watch Bands' AE1200 guide.
Duro MDV106 22mm: CNS Watch Bands' Duro guide (22mm, confirmed by multiple independent reviews and owner measurements), corroborated by WristyBizzle's review (22mm lug width, 48.5mm lug-to-lug), Blade Reviews' caliper measurement (22mm, noting most websites wrongly say 20mm), and Barton Watch Bands' MDV-106 22mm listings.
CA53W 20mm, Wave Ceptor 20mm, Edifice 22mm, G-Shock proprietary widths and the GA-2100's 16mm connector: Watchband.direct's Casio compatibility guide, corroborated by the GA-2100 row in our lug width chart research (CNS and Vivir).
MTP/LTP standard 18/20/22mm widths: Fishgoo's Casio strap guide, corroborated by NYWatchStore's Casio band range (18 to 22mm across the standard-lug line).
G-Shock proprietary attachment (no standard spring bars on most models): Watchband.direct's Casio and G-Shock guides, corroborated by CNS Watch Bands' note on the DW5600's fixed resin attachments.