Marianne, that is a really interesting question, but alas, not an easy one to answer. The amount of fabric will depend the requirements to make a certain pattern. Any pattern will list the fabric needed as well as the number of strips, templates or pieces. If you are considering designing your own pattern, you will have to figure how many blocks, the fabric needed per block, and what if any borders you will need.
I can demonstrate using your measurements of 57x70". Let's say the usable width of the fabric is 43". You can make a quilt by using the fabric as a panel and surrounding with borders to equal the size you want. Therefore, 57-43= 14" Divide by 2 to get 7" borders for side. You will need to cut your borders 7.5" to compensate for seam allowance. The length of your center panel would be figured by taking the finished length of desired quilt 70" and subtracting the amount for borders 14" = 56". The center panel fabric would then be 1.5 yards. You would need approx. 6 strips of fabric cut 7.5" for the borders. This would equal 45" or another 1.5 yards of fabric for the borders. So, as you can see, only 3 yds of fabric will infact make a quilt top. Unfortunately, that isn't the end of the story. There is the backing and binding to consider as well as any pattern and fabric changes.
Each quilt requirement is unique, but is really based on simple math. I keep a dollar store calculator in my sewing room. Trust me, it well worth the buck. LOL Hope I have helped answer your question. Come join us on the Quilter's Board. There are many knowledgable people willing to help.