Measure Twice, Cut Once
Lay the quilt top on a flat surface. To determine the border strip width to cut, decide the width you want the finished border to be, then add 1/2" for seam allowances.
The length of the side borders is determined by measuring through the center of the assembled quilt center, from top to bottom, and adding a 1/2" seam allowance.
To determine the length of the top and bottom border, measure through the center of the quilt from side to side, remembering to add the side border measurements plus the 1/2" seam allowance. To make it easier, sew on the side borders first.