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How to Measure Lug Width & Strap Size

The Quick Answer

To find your watch strap size, measure the lug width: the distance in millimeters between the inside edges of the two lugs, the arms that hold the strap. Use a digital caliper or a metal ruler. Most watches fit an 18mm, 19mm, 20mm, 21mm or 22mm strap. Not sure, or want to skip the measuring? Check your watch on StrapMatch.me, then shop by size at OPTO WATCH CO.

Choosing the right strap starts with one simple step. This guide walks you through it, so you get the right fit the first time.

1. What Is Lug Width?

Lug width is the internal distance between the lugs. The lugs are the two small arms that stick out from the watch case and hold the strap in place. Lug width is measured in millimeters, and it decides which strap size fits your watch.

Get it wrong by even 1mm and the strap will either not fit at all, or it will sit loose and leave a visible gap. That is why every strap is sold by lug width, and why it is the first number to know.

2. How to Measure Your Lug Width

You only need two minutes and one tool. A digital caliper is the most accurate. A metal ruler with millimeter markings also works.

  1. Take the watch off and lay it face up on a flat surface. Remove the old strap if you can.
  2. Find the space between the lugs, at the end where the strap attaches.
  3. Measure from the inside edge of one lug to the inside edge of the other. Never measure the outer edges of the lugs. That is the most common mistake, and it gives you a number that is too big.
  4. Read the number in millimeters. Round to the nearest whole size, such as 18, 19, 20, 21 or 22mm.
  5. Check both ends. The top and bottom lug gaps should match.

Tips for accuracy:

  • If you have no tool, measure the width of your current strap where it sits between the lugs. It is usually printed on the underside.
  • Ruler readings can be off by a hair. If the number lands between two sizes, measure again or use a caliper.
  • Measure the lug gap, not the width of the case.

3. Skip the Measuring with StrapMatch.me

Measuring is easy, but sometimes you cannot get a clean reading: a curved case, a tight lug gap, or a watch you do not own yet. That is what StrapMatch.me is for.

StrapMatch is our AI-powered strap sizing tool. It helps you find the right strap size and fit for your watch, so you can shop with confidence instead of guessing. It is a fast way to double-check your own measurement before you order.

A simple way to use both:

  1. Measure your lug width using the steps above.
  2. Confirm the size for your watch on StrapMatch.me.
  3. Jump straight to the matching size collection below.

4. Common Lug Widths

Watch straps are sold in whole millimeter sizes. These are the five sizes we stock at OPTO WATCH CO., each linked to its own collection.

Lug width

What it typically suits

Shop

18mm

Smaller and slimmer watches, many dress watches

18mm watch straps

19mm

Mid-size watches, popular on vintage-style pieces

19mm watch straps

20mm

The most common size, from everyday to sport watches

20mm watch straps

21mm

Less common mid-to-large watches

21mm watch straps

22mm

Larger watches, divers and chronographs

22mm watch straps

The "typically suits" column is a guide, not a rule. Your measured lug width always comes first.

5. Strap Width vs Buckle Width

You will see straps listed with two numbers, such as 20/16 or 22/18. The first number is the width at the lugs. The second is the width at the buckle, where the strap tapers. Your lug width decides the first number. The taper is a design choice that keeps the strap comfortable and neat on the wrist.

When you shop, pick the size by the first number.

6. How Lug-to-Lug Affects Strap Length

Lug width tells you which strap size fits. Lug-to-lug tells you how the watch will sit on your wrist and how much strap length you need. They are two different measurements, and both matter.

What is lug-to-lug? It is the distance from the tip of the top lugs to the tip of the bottom lugs, measured straight across the case, in the direction of the strap. Measure it with a caliper or a ruler, tip to tip.

How it changes strap length: the watch head takes up part of the loop around your wrist, and the strap fills the rest. So the longer the lug-to-lug, the less strap you need for the same wrist. The shorter the lug-to-lug, the more strap you need.

Here is a simple example, using a 170mm (17cm) wrist. The numbers are approximate, because strap thickness and lug shape change the result slightly.

Lug-to-lug

Strap length needed around the wrist

46mm

About 124mm

50mm

About 120mm

54mm

About 116mm

Each extra millimeter of lug-to-lug means about one millimeter less strap.

How it affects fit and comfort:

  • Long lug-to-lug on a small wrist: the lugs can overhang the edge of your wrist, and the strap cannot wrap snugly. As a rule of thumb, the lug-to-lug should not be wider than the flat top of your wrist.
  • Short lug-to-lug on a large wrist: you will need a longer strap, so check the strap length before you buy.
  • Thick or padded straps: they use up a little more length at the lugs, so allow for it.

Before you order, measure your wrist, subtract your lug-to-lug, and compare the result to the strap length on the product page. If you are unsure, StrapMatch.me can help you confirm the fit for your watch.

7. Shop by Size and Material

Once you know your size, OPTO WATCH CO. offers 18mm, 19mm, 20mm, 21mm and 22mm straps across our main materials:

Start with your size collection above, then pick the material that suits your watch and your routine.

8. What About Proprietary Lug Systems?

Some brands use custom lug designs that only accept their own straps, so a standard size cannot be measured and matched. Common examples include:

  • Apple (Apple Watch)
  • Audemars Piguet (Royal Oak)
  • Richard Mille
  • Cartier (Santos)

Always check whether your watch uses a proprietary system before you buy a new strap. If you are unsure, look up your model on StrapMatch.me first.