Hi Sallyanne,
I didn't see this before as I have been in Moldova. We have replaced the sisal at the scratching post one time and have the rope for the second replacement at home. I was surprised at what a great length you need but it is obvious:
if a post is 4 in wide and 20 in high, and the rope is 1/2 in thick you'll need roughly 40 circles around the post.
And each circle has a length of 2*pi*r, r being the radius of the post, that would be 2 in - that makes roughly 12.5 in per circle.
So the length of the rope needed will be 502 in that is 42 ft.
If your rope is thinner it will be more

I am sure the kitties will love their new post