if the backing fabric is 108" wide- 3 yards would give you another 108"...your piece would be 108 X 108. how large is your 'queen' sized quilt? 2 1/2 yards gives you 108" x 90"...just figure out how large the quilt is, decide how much extra you need for your backing (generally about 10" or so to make up for shrinkage ect) then divide that amount by 36",,, the amount of a yard- the answer will tell you how many yards you need. (if your quilt measures 93" x 100" I would buy 3 yards- the extra gives you enough + a little left over