I love using wide backing. In this case, I would buy enough fabric so that I could remove the selvage and add a 4 inch 'sacrifice' strip on each side. So Just to be safe I would buy 4 yards. That is plenty but also gives you wiggle room in case the cutting is not exactly square.

Whichever way you decide on, don't forget fabric for the binding (if you are planning to use backing fabric for the binding).