Size Guideline

In this page, I will split the guideline into 2 big parts, for standard shape (applicable for straps with straight shape at the lug area) and for certain brands (I will talk about Rolex, Audemars Piguet, Patek Philippe, and Hublot here).
First, we need to know some basic things. Please keep on reading.
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 lug area (please see the picture above).
Measure this in millimeter (not inch) for more accurate sizing. Width is expressed like this: 24/22. 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.
You might be confused, what is straight and tapered.
Straight is when the lug width and buckle width is the same number (example : 24/24)
Tapered is when the straps get tapered from lug width to buckle width (example : 24/22)
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 difficult to determine the lug 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

All Series

28mm tapered to 24mm

Rolex

Submariner, GMT Master II, Daytona, Explorer II

20 mm

Rolex

Deep Sea

21 mm

Tudor

Blackbay

22 mm
Omega Speedmaster Professional 20 mm
Tag Heuer Aquaracer 43mm  21 mm

Length :
There are two lengths required, the tail end (length of long section, at position 6H), and the buckle end (length of short section, excluding buckle, at position 12H). 
Length is expressed like this: 125/75. The first number (125) is the length in milli meters of the tail end with holes. The second number (75) is the length in milli meters 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 (do it tightly, don't loosely). 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 excellent place to start for help you decide your straps length (for standard shape straps). 
4.5 - 5.0” wrist (115 - 127mm) = 100/60mm 
5.0 - 5.5” wrist (127 - 140mm) = 110/60mm
5.5 - 6.0” wrist (140 - 150mm) = 115/65mm
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).
 
Now, let's talk about certain type of watches.
  • Rolex
If you previously wear Rolex with metal bracelet, and want to switch to leather straps, don't use the size guideline above!
You just need to count the number of links on your metal bracelet and give that information to us. That's it.
Note : Please don't count the end-links.
  • Patek Philippe 
Measure your wrist with string/thread (just like the guideline above), and then use this chart below (only for Patek Philippe)
105/70mm for 14,8 to 16,3cm (5.8 to 6.4") wrist (Size S)
115/75mm for 16,3 to 17,8cm (6.4 to 7") wrist (Size M)
125/80mm for 17,8 to 19,5cm (7 to 7.7") wrist (Size L)
 
  • Hublot 
Measure your wrist with string/thread (just like the guideline above), and then use this chart below (only for Hublot)

100/75mm - for 14.5cm to 15.5cm [5.7 to 6.1 “] wrist [size XS]

105/80mm for 15.5cm to 16.5cm [6.1 to 6.5 “] wrist [size S]

110/85mm for 16.5cm to 18.5cm [6.5 to 7.3 “] wrist [size M]

115/90mm for 18.5cm to 19.5cm [7.3 to 7.7 “] wrist [size L]

125/95mm for 19.5cm to 21cm [7.7 to 8.3 “] wrist [size XL]

 

  • Audemars Piguet ROO 42mm and Royal Oak
Measure your wrist with string/thread (just like the guideline above), and then use this chart below (only for Audemars Piguet ROO 42mm and Royal Oak)

105/55mm - for 14.8cm to 15.8cm [5.8 to 6.2 “] wrist [size XS]

105/65mm for 15.8cm to 16.5cm [6.2 to 6.5 “] wrist [size S]

105/72mm for 16.5cm to 17.2cm [6.5 to 6.8 “] wrist [size M]

115/75mm for 17.2cm to 18cm [6.8 to 7.1 “] wrist [size L]

125/80mm for 18cm to 19.5cm [7.1 to 7.5 “] wrist [size XL]

 
  • Audemars Piguet ROO 44mm
Measure your wrist with string/thread (just like the guideline above), and then use this chart below (only for Audemars Piguet ROO 44mm)

105/65mm - for 14.8cm to 15.8cm [5.8 to 6.2 “] wrist [size XS]

110/70mm for 15.8cm to 16.5cm [6.2 to 6.5 “] wrist [size S]

115/75mm for 16.5cm to 17.2cm [6.5 to 6.8 “] wrist [size M]

120/80mm for 17.2cm to 18cm [6.8 to 7.1 “] wrist [size L]

125/85mm for 18cm to 19.5cm [7.1 to 7.5 “] wrist [size XL]

 
Hope you understand it better after reading this page, but if you still confused, feel free to contact us!