I try to find easy ways to do things because at heart I am lazy! Mitred corners are one of those things that I've found an easy way to manage. Not saying it is the "correct" way but it works for me. I don't plan on entering a quilt in juried show; I just want to make beautiful quilts and I think mitred corners are a nice finishing touch. If there are several borders it can get a little tricky lining everything up and that equals stress. I've got more than enough stress in my life so I figured out a stress-free method of doing a mitred corner. I'll give you the math to figure out the length of the border strips but first I'll explain the mitre technique.
I prepare the borders in the usual way. When I stitch the borders to the quilt top I start and stop ¼” in from the edge of the quilt top and then press the seam towards the border. To mitre the corners I lay the corner of the quilt on my ironing board and press so that everything lies flat and smooth. Then I fold under one edge of the border to create a 45o angle and press. Make sure that the short edge of border that is folded under lines up with the long side of the border. I pin the folded edge in place. If there are multiple borders I am able to make slight adjustments so that each border seam matches up perfectly. In the example below I have a 1" green border, ¼” gold border and a 6" outer border. Notice how everything matches up perfectly.
To finish the mitre I use a thread that blends with the border fabric and using tiny blind stitches I sew the mitred corner in place from the right side of the quilt. If you use a very lightweight thread like The Bottom Line, Invisifil or Decobob the stitches will completely disappear. If there are multiple borders and the colours are very different use a different thread colour to match each border. One the mitre is stitched down you turn the quilt over and trim away the excess border fabric
Now here comes the math for the width and length of the border. You need the measurements of the finished quilt top without the seam allowances. Lets say the quilt top measures 60 ½” by 70 ½” with seam allowances so that means it will measure 60 by 70 without seam allowances. To this I want to add a border that is 8" finished (8 ½” with seam allowances). This means the finished width of the quilt with the borders will be:
The finished length of the quilt with the borders will be: