In a perfect world, this is what you should be able to do if both pieces of fabric were cut perfectly square. I would definitely buy a little extra for a fudge factor next time. I feel much more comfortable when my backing is at least 4" bigger all around than my top. I can deal with 2" bigger but I get very, very nervous toward the end.
I deducted a couple inches for the selvages but not for the seams.
3 yard quilt backing
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