I like to allow three inches on each side. I would buy 21 inches x 46 inches. That allows 3 inches on top, 3 inches bottom, 3 inches right side, 3 inches left side. I hope that makes sense. That is what I am comfortable with for framing. Other stitchers may use different measurements, but that is enough fabric that I feel comfortable. I have shot myself in the foot more than once trying to "save" on material. However, thank goodness, it was before I stitched the project!!!! Peace. Pam I have to remember the rule:Measure twice, cut once.