Watch size is a combination of strap length, watch case size and width of the lugs, or the metal pieces that are attached to the watch case in which the strap fits between. If you have a watch that fits, you can measure the watch band length easily to find a replacement strap or to determine the watch size you need when purchasing a new time piece. If you don't have a watch that fits, you can determine the watch size you need by measuring your wrist.
Turn your watch over so that the face of the watch is facing downward. Look for a number imprinted on the watch band. This number states how many millimetres wide the strap of your watch is.
If there is no measurement marking on the band, place your watch on a table with the watch face pointing upward. Locate the lugs on the bottom side of the watch face. If you have an analogue watch, these lugs will be located near the numbers five, six and seven.
Using a metric ruler, measure the distance between the insides of the two lugs on the bottom of the watch face. If you're measuring the lugs with the watch band still on, be extra careful to note the lug width and not the band width. This is especially important on watches that don't have the original or correct watch band on them.
Place your watch on top of an English ruler. Measure each section of the watch's strap in inches. Don't include the buckle length in your measurement.
If you don't have a strap that unbuckles, take a piece of ribbon and wrap it around the circumference of the band. Cut the ribbon so that it is the same length as your current watch band. Lay this piece of ribbon on top of an English ruler and note its measurement in inches.
Compare the length of your watch band to a band length conversion chart. This chart will tell you if the band length you need is short, regular, European standard, long, extra-long or extra-extra long.
Place your watch upright on an English ruler so that the watch face is facing upward.
Note where the watch case, or the part of your watch that holds the watch face, is. The lugs of your watch are included as part of your watch case.
Measure the length of the watch case in inches. Use the holes on the inside of the lugs, in which a pin is placed to keep the watch band on, as the starting and stopping point for your measurement.
Cut a 12-inch section from a piece of ribbon. Wrap this piece of ribbon around the circumference of your wrist.
Overlap one end of the ribbon over the remaining section of the ribbon that is not wrapped around your wrist. Mark the spot where the end of the ribbon meets the overlapping section with a permanent marker.
Using an English ruler, measure the length of the piece of ribbon between the end and the mark you made.
Take the measurement you got from measuring the band length and the measurement you got from measuring the watch case and add them.
Compare this total measurement with the one you got from measuring your wrist. These measurements should be very close, if not exactly the same.
Purchase a new watch that has the same total circumference as your previous watch and wrist length. If you're looking at watches with a band made up of many links, any number of links can be taken out or added to create the proper size. If you're looking at watches with a band that buckles, the holes in the band give some leeway for adjusting the watch to the proper size.