21mm watch straps are somewhat rare and harder to find than other strap sizes. Thus I’ve found that the selection of styles and materials in this size is not as great as in some of the other more typical watch band sizes. But they can be found, and several examples available online for ordering are shown below:

Was 21 printed on the current strap you have and/or was 21 arrived at by measuring between the watch lugs? If the appropriate style isn’t available in 21 mm online it may be possible to get another close size of watch strap such as a 22 mm width and either try to squeeze it in to fit between the lugs or trim it to make it fit (assuming the watch band is not made of metal) or to get a 20 mm and tolerate a somewhat unsightly gap. Though those approaches are definitely not recommended. For a watch with 21mm between the lugs the best option is generally to get the properly sized 21mm watch strap to fit the watch.

Nylon is a good choice in watch band material given it is strong, rugged, and comfortable. Below please find several 18mm nylon watch bands which can be ordered online.

Nylon watch bands are typically a good match for somewhat casual watches, and not as good a match for flashy or more formal watches such as a diamond or gold watch. Hope the page helped you find a the 18mm nylon watch band you’re looking for.

Below please find a selection of 26mm watch bands available on Amazon. The 26 mm size is sometimes found on larger watches, though it’s always important to measure the distance between the watch lugs with a metric ruler to be sure you’re selecting the right watch band size.

The length of the watch band is also a consideration, and this depends on wrist size. The placement of the band holes also affects the wrist sizes the band can accommodate. Good luck in your search for 26mm watch bands.