Not only is it just more convenient to know how much rope you'll need, but if you're commissioned for a custom piece, it is essential that you account for the cost of rope when you provide a quote.  Regularly estimating too low and having to cover this cost out of your own pocket is a sure way to make an unviable business for yourself.

So, here's what I can tell you about estimating rope quantity.

It depends on the kind of design you're planning; it depends how tight or loose your knot design is; and it depends on the size of your rope.

But, don't fret, here's a general guide based on how I estimated certain projects, and also how much rope I actually did use for others.

Example 1:  5 feet wide x 6 ft high "panel" type macrame wall hanging.  For this project, I used my 4 mm 3-strand rope. 

First, assume each strand will take up approximately 1" across the dowel. 60 inch dowel = 60 strands of rope (give or take a few depending on how close you'll place them on the dowel so let's say 64).  Again, this is just an estimate, but you'll want to round up to cover your costs.

Second, I knew I wanted to have the final piece 6 feet high/long (72"), and lots of my knots would be fairly close together, taking up a lot of rope real estate.  So I figured 72" doubled over is 144" then I went about 4x that because I wanted to be extra certain that I would have enough length to work with.  144" x 4.5 = 648" so I assumed 64 strands that were each 648" long = 41,472 total inches = 3,456 feet. 

In the end, this worked out well and I had about 12-18" of cut off in some places, and less in others, but in my mind, this is perfect because I was at ease as I worked knowing I would have enough and wouldn't have to add cord, which I personally don't like doing at all.  And saving myself the heart palps and paranoia while working was well worth adding to my scrap hoard at the end.

Example 2:  Macrame Hoop Chair with 31" diameter

This project uses 24 strands of 1/4" cotton rope -- either braided or 3-ply.  Remember, you want to allow a few inches extra for the dip in the seat to make sure your bum is comfy, as well as allowing room for the knots.  I always cut each strand 24 feet (so that's 576 feet total) for each hoop chair project, which is more than enough.  If you're budgeting, you'll be safe with ordering 500 feet of rope.

Example 3: 36" long macrame plant hanger (measured from the top ring, to the base of the plant pot) -- length of tassel at the bottom is up to you, but these measurements allow for at least a 12" tassel.

8 strands of rope, each 11 feet long, and I used my 8 mm single twist cotton string for this particular plant hanger.  But, for plant hangers in general, functionally you can use 3 mm, 5 mm or 8 mm single twist string -- they are all plenty strong -- so the size you choose just depends on the aesthetic you're looking for.  This particular plant hanger is very quick to make and doesn't use many knots.  The gathering knots at the top and very bottom under the pot in different colors provides a nice accent.