21~25mm. 1. Watch matching should pay attention to the principle of “appropriate size”. 1. Strap length: including long side and short side: for example, 115mm-75mm refers to a strap with a long side of 115mm and a short side of 75mm; it may also be the same length on both sides. 2. Tape head width and tape tail width: The tape head width is generally wider than the tape tail, such as 19mmX16mm, which means the tape head is 19mm and the tape tail is 16mm. There are also equal-width straps, such as 20mm. 3. The thickness of the strap: it may be the same thickness, or it may gradually become thinner, such as 4.5mm to 2.5mm gradually thinning, which means the thickness of the strap is 4.5mm from the thickness of the strap to the thickness of the tail 2.5mm, which gradually becomes thinner. 4. The lug width of the watch is fixed, which determines the width of the strap head. 2. Watch strap matching skills 1. Watch strap width: The standard ratio of watch strap width to dial diameter should be between 0.5 and 0.6 (based on the golden section, slightly fluctuating depending on the thickness of the wrist) 42mm dial diameter with 21-25mm width The strap is suitable. 2. Thickness of the strap: If the strap is too thin, the watch will look thin overall; if the strap is too thick, it will look bloated and not capable enough. The thickness of the strap head should be 1/4~1/3 of the thickness of the watch itself, which is more harmonious. 3. Length of the strap: If the wrist is thicker, it will be longer, if the wrist is thinner, it will be shorter.