If you intend to replace your watch strap,all straps are sold in mm the easiest way is to buy a strap guide,if not my tip is to cut off the old strap,using a craft or stanley knife.
Then take a ruler and measure inside where the strap pin sits,this will then determine the size strap you require ie 14mm 22mm etc. Some watch manufacturers have spring pins and some use screws.
If your watch is a screw type always use a very small amount of loctite or simmilar thread lock to stop the screws from working loose.
If your watch has a spring pin type please be carefull as these can fly out and easily get lost ,when purchasing a new strap try to buy one with new pins as they are prone to bending with age.
If you own an exspensive brand all parts are interchangable to fit the named parts to your new strap .
There are plenty of tools to carry out these tasks on ebay and save yourself a packet in todays climate every little helps.