Simply, measure the circumference of the two points you will be hanging the line onto, and add that number to the distance between the points. It is better to overestimate the length of cable you will need as there are no extensions for the zipline. In the case of excess cable, you can cut off the extra that you are left with, or wrap it safely around the tree. Circumference of Tree #1 + Circumference of Tree #2 + Distance between Tree #1 and Tree #2