The measuring sequence
Start where the watch sits.
A flexible tape is useful. A strip of paper and a ruler also work. Avoid elastic cord: it can stretch while you mark it.
01Wrap without pulling.
Place the tape around the wrist at the position where you normally wear a watch. Keep it in contact with the skin without making an indentation. Record the reading in millimeters and note which wrist you measured.
A reading taken across the wrist bone can differ from one just above it. The point is not to find the smallest number. It is to describe the place the watch will occupy.
02Mark the meeting point.
With a paper strip, mark exactly where the end meets the wrap. Lay it flat and measure from that end to the mark. Do not measure to the end of a loose overlap; that includes paper that was never part of the circumference.
For example, a mark at 160 mm means a measured circumference of 160 mm, or 16 cm. Keep that raw measurement. Do not add a guessed comfort allowance before sending it.
03Check the reading again.
Take the wrap off completely, then repeat it. If one reading is 158 mm and the next is 165 mm, check the tape position and tension rather than averaging the two into an apparently exact answer.
Your wrist can change through the day. If the watch is intended for long daily wear, note when a present strap feels tight and when it starts to move.
04Describe the preference.
Say whether you like the watch held in one place or with a little movement. Circumference alone does not tell us that. If a current watch fits well, its model and fastening position offer a useful comparison.
Do not tighten a strap until the case stops moving at any cost. Pressure, pinching or numbness are reasons to loosen it and reconsider the fit, not evidence of a secure adjustment.