Guide
Watch Strap Sizes Explained
A strap's size is its width where it meets the watch, in millimetres, and it has to match the gap between your watch's lugs. Here's how the sizes work, which watches use which, and what to do when yours is an awkward one.
Strap size is the same thing as lug width
When a strap is sold as “20mm”, that figure is its width at the end that attaches to the watch. It must match your watch's lug width: the inside distance between the two lugs that hold the spring bar. A 20mm lug width needs a 20mm strap. Case diameter, lug-to-lug length and wrist size don't change the number you buy.
If you don't know your lug width, look your watch up in the Strap Index finder or measure it with our on-screen ruler guide.
Why a single millimetre matters
Too narrow and the strap leaves a visible gap at each side, slides along the spring bar and wears the strap's edges. Around half a millimetre narrower is usually unnoticeable; a full size down is obvious on leather but often fine on a NATO.
Too wide is worse. The strap either won't go between the lugs at all or has to be squeezed in, which makes it bunch and pushes against the spring bar tips. That's how bars pop out. Never force a wider strap into narrower lugs.
The common sizes, and the watches in our index that use them
| Width | Typically found on | Watches in our index | Examples |
|---|---|---|---|
| 18mm | Small and vintage-sized watches, slim dress watches around 36–38mm, and classic digital Casios. | 5 | Seiko Seiko 5 (SNK), Vostok Amphibia 710, Casio F-91W |
| 20mm | The most common size: 38–42mm sports, dive, field and dress watches. | 36 | Seiko Prospex 62MAS, Seiko Prospex Willard, Seiko Marinemaster 300 |
| 21mm | An odd size used on some modern dress and sports watches. | 4 | Grand Seiko Snowflake, Rolex Datejust 41, Tissot Gentleman |
| 22mm | Larger divers and sports watches, and several smartwatches with standard lugs. | 15 | Seiko SKX007 / SKX009, Seiko 5 Sports (5KX), Seiko Prospex Turtle |
Below 18mm you're mostly into women's and small vintage watches. Above 22mm (24mm, 26mm) you'll find oversized pilot and Panerai-style watches.
A useful rule of thumb (and its limits)
Lug width tends to be about half the case diameter. Across the watches with standard lugs in our index, it averages 50% of the case diameter, so a 40mm watch usually takes a 20mm strap. But it's only a guide: the Seiko Prospex Sumo and the Vostok Amphibia 710 both have narrower lugs than their case size suggests. Always look up or measure; never guess from the case size.
Tapered straps: two numbers on the label
Many leather straps narrow towards the buckle. They're listed with two widths, such as 20/18mm or 20/16mm. Only the first number, the lug end, has to match your watch. More taper looks dressier and slimmer; a straight, untapered strap looks sportier and suits dive and tool watches. NATO and perlon straps are the same width all the way along.
Odd sizes: 19mm and 21mm
Most watches use even widths, but some use 19mm or 21mm. Straps in those sizes are harder to find, and an even-sized strap is a compromise. We cover your options in 19mm and 21mm watches: finding straps for odd lug widths.
What about smartwatches?
Some smartwatches use ordinary spring bars and standard widths. Samsung Galaxy Watch 6 (44mm) and Samsung Galaxy Watch (46mm, Gen 1) and Garmin Forerunner 265 all take normal straps. Others use a proprietary slot or clip that needs model-specific bands or an adapter. See integrated, proprietary and smartwatch straps.