How to Measure Your Watch's Lug Width
Every strap purchase starts with one number. Get it right and the strap sits flush against the case like it was made for the watch. Get it wrong by a single millimetre and you either can't fit the spring bar at all, or you end up with a visible gap on both sides that you'll never stop noticing.
Here's how to find that number in about a minute.
What lug width actually is
The lugs are the four horns projecting from your watch case that hold the strap. Lug width is the distance between the two lugs on one side — the gap the strap slots into, measured in millimetres.
It is not the width of the case, and it is not lug-to-lug distance. Lug-to-lug measures the case from the top of the upper lugs to the bottom of the lower ones, which tells you how a watch wears on your wrist but nothing about what strap fits it. People mix these up constantly, and the numbers are wildly different — a watch with 20mm lugs might have a 48mm lug-to-lug.
Method 1: Digital calipers (the reliable way)
If you own calipers, this takes ten seconds.
- Remove the existing strap or bracelet.
- Open the jaws and slide them into the gap between the lugs.
- Close gently until they touch both inner faces.
- Read the number.
You'll almost always get a whole millimetre — 18, 19, 20, 21, 22. If you're reading 20.4mm, you're probably catching a chamfer or a bit of the lug's inner curve. Reseat and try again.
Digital calipers cost about £15/$20 and are worth owning if you swap straps at all.
Method 2: A ruler (good enough)
No calipers? A metal ruler with millimetre markings will get you there.
Lay the ruler across the gap between the lugs and read the distance from the inside of one lug to the inside of the other. Do it in good light, and check twice.
A ruler with a printed edge margin is harder to line up accurately, so use one where the scale starts flush with the end. If you land between two numbers, round to the nearest whole millimetre — lug widths are essentially never fractional.
Method 3: Look it up
Most manufacturers publish lug width in their technical specifications. Search your reference number — not just the model name, since the same model line often changes lug width across generations. A 40mm dive watch and its 41mm successor frequently differ by a millimetre.
If you still have the original bracelet or strap, you can also measure the end that sits between the lugs. Measure the very end, not further down: most straps taper, so a strap measured at the buckle will read narrower than its actual lug width.
Four mistakes that cause wrong orders
Measuring the outside of the lugs. You want the gap between them, not the total width of the case at that point.
Measuring a tapered strap in the wrong place. A 20mm strap that tapers to 16mm at the buckle is still a 20mm strap. Only the end that meets the case counts.
Assuming even numbers. 19mm and 21mm are real and more common than people expect — several Rolex and Grand Seiko references use them. If your measurement keeps landing on an odd number, trust it.
Confusing lug width with case diameter. A 42mm watch does not take a 42mm strap. Case diameter and lug width are unrelated measurements.
What if you land on 19mm or 21mm?
Odd lug widths used to be a genuine problem — most strap makers only produced even sizes, leaving owners of certain references to either force a 20mm strap into a 21mm gap or live with a bracelet forever.
A well-designed single-pass strap solves this differently. Because the material has some give and the strap passes underneath the case rather than terminating in a rigid end, a strap built for 20mm seats correctly in a 19mm gap, and a 22mm seats correctly in 21mm. No forcing, no gap.
That's how MonoPass sizing works:
- 20mm — fits 19mm and 20mm lugs
- 22mm — fits 21mm and 22mm lugs
Two sizes cover four widths.
Common lug widths
A starting point for some frequently asked references. Always confirm against your own watch or the manufacturer's specification before ordering — lug width changes between generations of the same model.
| Watch | Lug width |
|---|---|
| Tudor Black Bay 58 | 20mm |
| Tudor Black Bay 41 | 22mm |
| Seiko SKX007 / SKX009 | 22mm |
| Seiko Turtle (SRP777) | 22mm |
| Omega Speedmaster Professional 42mm | 20mm |
| Omega Seamaster Diver 300M 42mm | 20mm |
| Rolex Submariner 116610 (40mm) | 20mm |
| Rolex Submariner 126610 (41mm) | 21mm |
Frequently asked questions
Can I fit a 20mm strap on 22mm lugs? No. It will physically attach, but you'll have a 1mm gap on each side and the strap will shift under the lugs. Order the correct width.
Can I fit a 22mm strap on 20mm lugs? Also no — it won't seat between the lugs at all.
Does lug width change how the watch fits my wrist? Not directly. Lug-to-lug distance and strap length determine fit. Lug width only determines which straps will attach.
What if my watch has an integrated bracelet? Some cases are designed so that only the manufacturer's bracelet will fit, with no conventional lug gap. Measure carefully — if there's no clear gap with a spring bar, aftermarket straps may not be an option.
Still not certain? Send us your watch's make, model and reference number and we'll confirm the lug width for you before you order.
