On my old recurves the string length is equal to the distance measured tip to tip of the unstrung bow. in other words when unstrung the string is held on by the nocks on each tip with just enough pressure to hold the string in place. maybe if you know the fistmele you can bend the bow parallel to a wall while a friend measures the fistemele and string length or like brice says go to a pro shop. Hope this helps some.