Measure the Old String
Hammer a nail into a board and loop one side of the old string on the nail.
Pull the string tight and hammer a second nail through the free loop to hold the string in place.
Measure the distance between the two nails to determine the length for your new string.
Measure the Bow
Lay the long bow flat on the ground. Have a friend hold one end of a tape measure at the tip of the bow. Pull the tape measure to the opposite end and hold it tight. Read and record the measurement.
Subtract 3-inches from the measurement to determine the standard string length for your bow.
Contact the manufacturer of the new string to ensure three inches is the preferred measurement for their material.