Strap and lug width finder

Measure the gap, not the old strap

Lug width is the distance between the inside faces of the lugs, where the spring bar sits. Use digital calipers if you have them. An old strap is a poor reference because leather compresses at the ends and rubber can swell, so a worn 20 mm strap often measures 19.4 mm.

Even widths dominate: 18, 20 and 22 mm cover most of the market. Odd sizes exist, particularly 19 and 21 mm on Swiss watches, and cost more because fewer makers stock them. A strap 0.5 mm too wide will not go in; one 1 mm too narrow will rattle.

Two further traps. Do not derive lug width from case diameter: the rule that it is about half is often false, and the Citizen Promaster Diver BN0150 is a 44 mm case with 20 mm lugs. And do not assume the brand publishes it. Longines does not print lug width for the Spirit, and on a Cartier Tank the strap runs between brancards rather than lugs, so there is no gap to quote.

A worked example: 20 mm lugs and a 7 inch wrist

You have a Longines Spirit in the 40 mm case. Longines does not publish lug width; third-party measurement gives 20 mm for the 40 mm and 21 mm for the 42 mm, which is why the two do not share straps. Your wrist measures 7.0 inches, 17.8 cm, or 178 mm. Enter 20, then 7 with inches selected.

1. Strap width: 20 mm. Buy the number, not something close.
2. Taper: 18 mm. The tool subtracts 2 mm and rounds to the nearest even number.
3. Buckle: 18 mm, matching the tail rather than the lugs.
4. Spring bars: 20 mm long, about 1.78 mm thick for a sports case, 1.5 mm on a dress case.
5. Length: regular, about 75/115 mm to 80/120 mm, since 178 mm is under the tool's 180 mm line.

Notice how close that is to the boundary. At 7.1 inches (18.0 cm) the same tool returns long, so if you are within a tenth of an inch of the line, take the longer strap: extra length hides in the keepers, and a short strap cannot be fixed. The order is a 20 mm strap, an 18 mm buckle if none is included, 20 mm spring bars and a tool. Indicative US prices, before sales tax: leather $25 to $150, rubber $30 to $120, nylon $12 to $30, bars $8 to $15, tool $10 to $25. Our page on strap and bracelet costs has the detail.

Taper, buckle and spring bars

Most straps taper toward the buckle, usually by 2 mm, so a 20 mm strap generally ends at 18 mm and takes an 18 mm buckle. Buy the buckle to match the tail, not the lugs. Spring bars are normally 1.78 mm in diameter for tool watches and 1.5 mm on thinner dress cases, and the fat ones will not fit narrow lug holes.

Quick release spring bars, with a small lever on the bar, make strap changes a five second job. They are slightly less strong than a solid bar, which matters on a dive watch and not at all on a dress watch.

At 14 mm and below the tool stops tapering and returns 12 mm, since straps that narrow are usually straight cut. Treat the column as the common case, not a specification: the strap listing is the authority and states both figures, as 20/18.

Our straps and bracelets guide sets out the diameters: around 1.5 mm on dress watches, 1.8 mm on sports watches, and roughly 2 mm and above on dive watches, where ISO 6425 requires the attachment to survive a specified pull test. Bar length must match lug width exactly. An undersized bar seats, looks right and shears under a knock.

Length, and why so many straps are wrong

Strap length is quoted as two numbers, the short side and the long side in millimeters, for example 75/115. A regular strap suits most wrists between about 6.5 and 7.5 inches (16.5 to 19.0 cm, 165 to 190 mm). Below that, look for short straps or you will run out of holes. Above it, look for long or XL, especially with a deployant clasp, which eats length.

Those thresholds are 155 mm and 180 mm of circumference. The tool converts what you type into millimeters first, at 25.4 mm to the inch and 10 mm to the centimeter.

The geometry is worth seeing once. A 75/115 strap is 190 mm end to end, and it wraps your wrist plus the case between the spring bars, perhaps 22 to 25 mm on a 40 mm watch. On a 178 mm (7.0 inch, 17.8 cm) wrist that leaves roughly 35 mm of tail past the buckle, about two keepers' worth. On a 165 mm (6.5 inch, 16.5 cm) wrist the tail is close to 50 mm, which is the flapping end people complain about and the reason short straps exist.

What the length suggestion cannot know

Circumference is one input to a problem with five. Treat the length row as a starting point.

• Case and strap thickness. There is no thickness field, and a 15 mm case on a 5 mm padded strap needs more length than a 9 mm case on a 2 mm dress strap.
• Single-pass straps. A NATO runs under the caseback, so thickness enters the path twice and the keeper layer takes more again. Buy longer than the two-piece figure.
• Deployant clasps. The strap folds into itself, consuming length and usually meaning a shortened tail side.
• Wrist shape. A flat wrist and a round one at the same circumference do not fit the same watch, which is why lug-to-lug distance outranks diameter and why the watch size calculator is this tool's companion.
• Time of day. Wrists swell, commonly by about 6 mm (a quarter of an inch) between a cold morning and a warm evening. Our answer on how to measure your wrist covers why the afternoon figure is the one to use, and why measuring over the wrist bone instead of just past it gives a number that fits nothing.

One quirk: the tool ignores any wrist below 110 mm or above 260 mm (4.3 to 10.2 inches, 11.0 to 26.0 cm). The box does not relabel when you switch units, so leaving it on centimeters and typing 7 gives no length row at all rather than a wrong one.

Cases the finder does not fit

• Integrated bracelets. No conventional lug gap, and the end piece is shaped to that case. Only the manufacturer's part or a made-to-order strap fits.
• Curved end straps. The end is cut to the curve of the case, so it must match the radius as well as the width.
• Vintage lug holes. Older cases can be drilled for thinner bars, so a 1.78 mm bar will not always enter. Our vintage buying guide covers what else changes.
• Drilled versus blind lugs. Drilled lugs take a pointed pusher from outside. Blind lugs need the forked end, a steady hand and taped lug faces.
• Odd widths. 19 and 21 mm are real sizes with real watches behind them, including the 42 mm Spirit. The tool handles them; the market barely does.

The decision here is a single order: a strap in the right width, a buckle in the taper width, spring bars and a tool. Getting the width wrong wastes the strap; getting the bars wrong can cost the watch, which is why a strap that keeps failing is worth diagnosing. For materials rather than sizes, see nylon, leather and metal compared.