Straps are measured in milimeters. You need to know two dimensions: width and length.
Width:
Width is the distance between the lugs, where the strap meets the watch. To determine your width, you can take 
a ruler or caliper, and measure the width of the part of the strap that uses spring bars / screws to attach to your watch case. Measure this in millimeters.Width is expressed, for example like this: 24/22 (24mm wide by 22mm wide). This first number (24mm) is the width of the strap between the lugs of the watch case. The second number (22mm) is the width of the strap at the buckle.
A “straight” strap is one that is the same width at the lugs and the buckle, i.e 24/24 (24mm wide at lugs by 24mm wide at buckle). While a “tapered” strap is one that tapers in width from the lugs to the buckle, i.e 24/22 (24mm wide at lugs by 22 wide at buckle).
There is also a table for several watches that are commonly use to change their strap. If you don’t find lug width of your watch below, and feel difficulty to determine the luv width, please tell us: the brand name, and model of your watches, or simply the pictures of your watches, and we will help you figure out.
Watches Brand | Series / Model / Watch Case | Lug Width |
Panerai | 40mm Luminor Case | 22 mm |
Panerai | 42mm Luminor Case | 22 mm |
Panerai | 44mm Luminor / Submersible Case | 24 mm |
Panerai | 47mm Luminor / Submersible Case | 26 mm |
Panerai | 45 or 47mm Radiomir | 26 mm or 27 mm |
SevenFriday | Any P-1, P-2, P-3, M Series | 28 mm |
Rolex | Submariner, GMT Master II, Daytona, Explorer II | 20 mm |
Rolex | Deep Sea | 21 mm |
Tudor | Blackbay | 22 mm |
Bell&Ross | BR 01, BR 03 | Add extra service B&R |
Audemars Piguet | ROO, Diver, etc | Add extra service AP |
Length:
There are two lengths required, the tail end (length of long section), and the buckle end (length 
of short section, excluding buckle). Length is expressed, for example like this: 125/75. The first number (125) is the length in millimeters of the tail end with holes. The second number (75) is the length in millimeters of the buckle end.
The strap length suggestion is based on wrist size. To determine the your wrist length, you can simply wrap a piece of string/thread around your wrist at the point where you normally wear your watch, mark the string where it crosses itself, lay it flat on a table and measure the distance between the marks. This is your wrist size.
The table below is an excelent place to start for help you decide your size length order.
6.0 - 6.5” wrist (150 - 164mm) = 120/70mm
6.6 - 7.0” wrist (165 - 178mm) = 125/75mm
7.1 - 7.5” wrist (179 - 190mm) = 130/80mm
7.6 - 8.0” wrist (191 - 203mm) = 135/80mm
8.1 - 8.5” wrist (204 - 216mm) = 140/85mm
8.6 - 9.0” wrist (217 - 229mm) = 145/90mm
However, don’t let this table be your strict rule. Choosing the strap length is all personal preference. There is no “right” or “wrong” in choosing them. A length that we consider “too short”, or “too long” might consider to be “just right” for you. Strap length is an art, and as your taste changes, your idea of what looks “just right” will probably change too. For example, If your wrist is 6.7”, you can also go with 125/75mm for a more conservative look (less tail, centered buckle), or you could go up to 130/80 for a longer look (more tail, buckle over to the side more).
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