Buying a replacement watch strap should be straightforward. Then the listings start throwing numbers at you: 20 mm, 20/18 mm, 120/80 mm. They describe different parts of the fit, and mixing them up is how a strap that looked perfect online ends up back in its envelope.

Here is the short answer: measure the space between your watch’s lugs in millimeters to get the strap width. Then check the strap’s length against your wrist or a strap you already wear comfortably. Check buckle width only if you plan to reuse a buckle or clasp. For an ordinary two-piece strap, those three checks will settle most sizing questions.

First, find the lug width

The lugs are the two pairs of arms that stick out from the watch case and hold the strap. The lug width is the gap between the inside faces of one pair. Measure the gap where the strap attaches, rather than measuring across the outside of the case. A 40 mm case, for example, does not tell you whether its strap needs to be 18, 20, or 22 mm wide.

A metric ruler can get you close; calipers make an awkward or odd-size gap easier to read. Take the measurement at the watch itself, particularly if the old strap has stretched, been cut, or might not be original. If the maker publishes a strap width for your exact watch model, use that as a cross-check. Timex’s band-size guide and Hirsch’s sizing guide both describe this inside-the-lugs measurement.

The number you find is normally the number to select on a replacement strap. A 20 mm lug gap calls for a 20 mm strap at the watch end. Do not assume every watch uses an even number: 19 mm and 21 mm fittings exist. Squeezing a wider strap between narrower lugs can distort the material and interfere with the spring bar, so buy the stated size unless the strap maker explicitly specifies another fit.

Then make sure the strap reaches around your wrist

Width answers will it attach? Length answers will it wear comfortably? On a conventional two-piece strap, the longer piece has the holes and the shorter piece carries the buckle. A listing such as 120/80 mm commonly means a 120 mm long piece and an 80 mm short piece, measured without the buckle. Some sellers show the short piece first, though, so read the labels rather than trusting the order of two bare numbers.

If your current strap fits, lay both pieces flat. Measure the longer piece from its attachment end to the tip and the shorter piece from its attachment end to the buckle end, excluding the buckle itself. Compare those measurements with the manufacturer’s sizing diagram, and note which hole you normally use. A new strap with similar piece lengths and hole positions is a sensible starting point. WatchGecko explains the common 120/80 notation, while Jean Rousseau’s measuring guide treats the two pieces as separate measurements.

No comfortable old strap? Measure your wrist where you actually wear the watch, then use the fit range for the specific strap you are considering. Do not simply add 120 and 80 and assume the result is a 200 mm wrist: the watch case, buckle, and available holes all affect the finished fit. Makers such as Barton publish wrist-fit ranges for their straps; those ranges are more useful than a generic “regular” or “long” label.

If you are buying it as a gift and cannot measure the recipient’s wrist, a photo of the underside of their current strap may reveal both the strap size stamp and the hole they use. Check that the photographed watch is the one you are buying for. When in doubt about length, choose a seller that states the wrist range and accepts returns; the correct lug width alone cannot guarantee a comfortable fit.

What does 20/18 mm mean?

That slash usually describes a taper. In 20/18 mm, the strap is 20 mm wide where it meets the watch and 18 mm wide at the buckle. The first number must suit the lugs. The second matters if you want to move your old buckle or a separate deployant clasp onto the new strap.

If the new strap comes with its own buckle and you are happy to use it, match the watch-end width first. If you want to keep a signed buckle, measure its inside opening and look for a strap with the same buckle-end width. A 20 mm strap that tapers to 16 mm may attach to the watch perfectly yet leave your 18 mm buckle unusable. If you’re reusing a deployant clasp, check the strap’s thickness and the clasp’s attachment design as well as its width; matching the width alone does not guarantee compatibility. Jean Rousseau’s strap listings illustrate why makers show both widths.

A few fittings need an extra check

These measurements describe the common straight-ended, two-piece strap. Quick-release spring bars change how you remove the strap, not the lug width it needs. Check the product listing for spring bars and enough clearance around the case. One-piece pull-through straps, including many NATO-style straps, still need the right width, but their overall length and keeper placement work differently; use that style’s own wrist-fit guidance. Metal bracelets, integrated straps, curved ends, and fixed lugs may require a watch-specific end link or attachment. For those, a matching millimeter width is necessary but may not be sufficient.

Before ordering, write down four things: your watch model, the measured lug width, the strap’s stated wrist range or two-piece lengths, and whether you need to reuse a particular buckle. That small note is more reliable than trying to remember which “20” a listing meant.

The best replacement strap is the one that meets the watch cleanly and fastens where you want it. Get the width from the watch, the length from the wrist, and the buckle width from the hardware you intend to keep. The numbers stop looking cryptic once each has a job.

The photographs and diagrams here are editorial illustrations. Measurements and compatibility should be confirmed against your watch model and the strap maker’s specifications before purchase.