We need to know how many blocks you have and if you intend to put in sashing before we can help with the size of the backing. if you have 12 inch blocks and putting 6 across would give you a width of 69.5 inches across.
(11.5 x 6 = 69 + .5 inch for the edge = 69.5.) To that I would add 6 inches = 75.5 inches which makes it easier to center the top on. Some people use less than 6 inches, and I have also, but prefer having the extra space.
You would do the same for the length as the width.
I never prepare the back until I have finished the quilt top, so I have the measurement of the actual top and not what I thought it would be.
Last edited by quiltingcandy; 04-15-2014 at 12:57 PM.