I found this info on another post:
here is a quote directly from the OBW book:
"How much fabric should you buy? There are very few rules, but there are some guidelines. The size of the quilt depends not only on how much fabric you buy, but also on how many hexagon blocks...you eventually use in the quilt...(and) on how you arrange t he hexagons and on the shape of the finished quilt. To make a good lap-size, twin, or full-size quilt, 4.5 y ards for hexagon blocks or 5.75 yards for octagon blocks is enough. For a queen size or larger quilt, double those amounts to 9 yards for hexagon blocks or 11.5 yards for octagon blocks.
Measure the repeat. If the (repeat) is 6" to 8", which is a short repeat, the fabric will produce a very small project. Larger prints usually feature a repeat about every 24". This is the type of fabric I typically choose. Four to five yards makes a very comfortable lap quilt, and depending on the borders used, it can grow to almost any size.
For Hexagons: 6 repeats at 24" each is exactly 4 yards...Add an extra half yard or yard (4. 5 or 5 yards) to have a piece of the original fabric as a reference."