There are a couple of ways to do this. a) Use your existing strap, lay it out flat and measure from button hole to button hole. b) If you don't have a strap, use a piece of string and tie one end to a button on your guitar. Sling the string around your shoulder and adjust the other end of the string to the other button on your guitar until you find the position you would like your guitar to be. Then measure the length of string from button hole to button hole. Please refer to my Guitar Straps page to learn more.