How to measure watch lug width for a new strap

A strap a millimetre too wide won’t go in; a millimetre too narrow rattles. Measuring watch lug width takes a ruler and about thirty seconds — here’s exactly where to put it, and the three cases where the number on the ruler still isn’t the whole answer.

Written by the developer of Loupe · Published 24 September 2026

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What lug width is — and what it isn’t

The lugs are the two horns that stick out from the case and hold the strap. A spring bar runs between them, and the strap threads over that bar. The lug width is simply how far apart those two horns are on the inside, and strap sellers use it as the size of the strap: a “20mm strap” is one that fits a 20mm gap.

Two other numbers get confused with it:

• Lug-to-lug is the length from the tip of the top lugs to the tip of the bottom ones — the footprint of the watch down your wrist. It decides whether a watch sits well on you. It has nothing to do with strap width.
• Buckle width is the other end. Most straps taper, and sellers write the pair as two numbers: a 20/16 strap is 20mm at the lugs and narrows to 16mm at the buckle. Only the first number has to match your watch; the second matters if you’re reusing a buckle or a deployant clasp, which has to match the buckle end rather than the lug end.

How to measure watch lug width, step by step

1. Take the strap off. If it has quick-release spring bars there’s a tiny lever on the underside of the strap near each lug: pull the lever towards the middle of the strap and the end drops out, no tool needed. Otherwise use the forked end of a spring bar tool, slid into the gap between the strap end and the lug, to compress the bar. If you don’t have either, skip to the next section.
2. Lay the watch face down on a soft cloth so the crystal and the case sides don’t pick up scratches.
3. Measure the inner gap. Put a millimetre ruler or calipers across the space where the strap sat, and read from the inside face of one lug to the inside face of the other. This is the measurement that matters — never the outside of the lugs, and never the lugs plus the case.
4. Round to the nearest whole millimetre. Straps are sold in whole millimetres, so a caliper reading of 19.94 is a 20. If you’re working from a ruler, aim to be sure within about half a millimetre; digital calipers take the guesswork out entirely.
5. Write it down with the rest of the watch’s details, so you never measure it twice. Our guide to cataloguing a watch collection lists what else is worth recording.

Can you measure it without taking the strap off?

You can, and for a quick check it’s usually close enough: rest the ruler across the watch where the strap meets the case and line the marks up with the inner edge of each lug. Two things to know before you order from that number. First, a fitted strap presses against the lugs and hides the inner faces, so the reading tends to come out slightly narrow. Second, you’re sighting across a curved case, which is easy to do a millimetre wrong.

There are two better shortcuts if the strap is staying on:

• Look at the strap you already have. Many straps have the size stamped or embossed on the underside, often as a pair like “20/16”. If the strap is the original one, the first number is your lug width.
• Measure the strap itself at the lug end rather than the gap. A strap that fits properly is the same width as the gap it sits in.

Both shortcuts assume the strap on the watch is the right one. On a second-hand watch it often isn’t, which is why the measurement with the strap off remains the answer worth trusting.

Common lug widths, and the half-the-case rule

Most watches land between 18mm and 24mm, and even numbers dominate: 18, 20 and 22 are the sizes every strap maker stocks, with 20mm the one most often quoted as standard for a men’s watch. Wider dive and pilot watches run to 24mm and beyond; small dress and vintage pieces drop to 16mm and below.

There’s a rule of thumb that lug width is roughly half the case diameter — a 40mm watch taking a 20mm strap, a 44mm watch taking 22mm. It’s a reasonable guess when you’re browsing, and it’s wrong often enough that it is not a substitute for a ruler. Case shape, lug design and a brand’s house style all move the number, and a 38mm case can be an 18mm or a 19mm depending on who made it.

What to do about odd sizes like 19mm and 21mm

Odd widths are real, not a mistake in your measuring. Several brands use 19mm, 21mm or 23mm, sometimes because a vintage reissue keeps the original dimensions, sometimes to keep owners buying the factory strap. Aftermarket choice at those sizes is thinner but it exists, and specialist strap sellers list them.

Strap makers do sometimes suggest sizing up by a millimetre in a soft material — a 20mm leather strap squeezed into 19mm lugs — and it can work at the cost of a compressed strap end. Don’t try it with anything rigid, and don’t force a metal bracelet into a gap it wasn’t made for.

When the gap measures 20mm and a 20mm strap still won’t fit

Three cases where the number alone doesn’t decide it:

• Integrated bracelets. On designs where the bracelet is part of the case line — the Royal Oak and Nautilus school of watch — a straight strap won’t fit even when the gap measures a standard width, because the end has to be cut to the case profile. Those watches need a strap made for the model.
• Curved end links. Plenty of ordinary lugged watches have a curved case flank, and a straight-ended strap leaves a visible gap either side. A curved-end strap made for that case closes it.
• Fat spring bars. Some dive watches, Seiko’s among them, use thicker spring bars — 2.5mm rather than the usual slimmer sort. The lug width is ordinary, but a strap whose spring bar channel was made for a standard bar won’t take one.

All three are decided by the model, not by the gap — which is where knowing exactly what watch you have starts to pay.

Knowing the model gives you the factory strap size

Once you have the brand, model and reference, the original lug width is usually a search away: brands publish it, and dealers list it with the specification. That is also how you find out whether the model wants a curved end or an integrated strap. Start with the reference number if it’s legible — here’s where each brand hides it — or with the markings on the caseback if the dial doesn’t say enough. If you can’t read either, identifying the watch from a photo is the way in.

Not sure what watch you’re measuring?

Photograph the dial and Loupe names the brand, model and reference with a confidence rating, plus an estimated resale range — clearly labelled as not an appraisal. Enough to look up the factory strap size before you order. Free trial, cancel in Google Play.