I would love to do a log cabin quilt does anyone know how much fabric I need its for a double bed
As a rule of thumb, unless your pieces are exceptionally small or really huge, I reckon that the top of a quilt will require twice the area of the finished quilt. That's to allow for all the seam allowances and a small amount of wastage.
So, for example, if your finished quilt will be 90" x 90" and the fabric is 45" wide you will need about 360" = 10 yards. The fabric is more likely to be 42" wide so compensate and reckon on 10m. That's 40 metric FQs (half metre x half width) or about 45 imperial FQs (18" x half width).