H ÷ D x C ÷ 1,000 = L1. Measure theheight (H) and the circumference (C) of the post in millimeters. 2. Choose which diameter (D) rope you want. 3. Divide the height (H) by the rope diameter (D) and multiply by the circumference (C) which gives the length in millimeters. 4. Divide this figure by 1,000 to convert the length (L) to metres.