1. Choose the right width
Measure the width between your watch lugs to make sure you’re buying the right width strap(where the watch band attaches to the watch). The correct width determines whether the strap can be perfectly paired with your watch; A strap that is too large will result in failure to install, and a strap that is too small will create unsightly gaps and annoying movement.
Lug width is measured in millimeters(usually an even number), which is why you’ll see sizes like 22mm and 24mm on websites. Get out a ruler and measure the distance between your lugs before you buy. You can also look up your lug widths on the manufacturer’s website or Google. Many straps have even sizes, and odd sizes are rare.The general consensus is that strap width should be approximately 50% of the watch case’s diameter.
2. Choose the right length
The length of the strap consists of two parts, the tail end (length of long section), and the buckle end (length of short section, excluding buckle). Length is expressed, for example like this: 130/80. The first number (130) is the length in millimeters of the tail end with holes. The second number (80) is the length in millimeters of the buckle end.
How to choose the length？
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.
For women, it is between six-and-three-quarters to eight inches, while men average between seven-and-three-quarters and nine-and-a-half inches.Of course this is not an absolute value, please refer to personal wrist measurement data for details.
After getting your wrist size, you can refer to the following data：
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
It should be noted that this is not a strict rule, you can choose the "just right" length according to your preferences and aesthetics.