This is a great free calculator, but it is not much help if you are piecing your borders. I don't usually piece my borders, sometimes I will put a block in each corner if I don't have quite enough length or actually make a pieced border of triangles, strips or blocks. But it is just a personal preference. Many quilters do not mind having seams in their borders.
http://vrya.net/quilt/flash/QuiltCalc_Loader.html