Sashing a quilt involves sewing strips of fabric between each quilt block to surround the blocks with vertical and horizontal strips of fabric. The desired effect with sashing is to set off the quilt blocks and help to enhance the patterns and colors in the quilt blocks. Figure and measure your quilt fabric for sashing a quilt in the same way you would figure fabric requirements for any quilt block.
Draw a diagram of your quilt as it will look with the sashing. Include the correct number of quilt blocks in the rows. For example, if your quilt will be five quilt blocks across and three quilt blocks down, draw 15 quilt blocks in a grid configuration. Note the dimensions of the quilt blocks to help you know what numbers you are working with.
Determine the width of the sashing strips. For example, if you want your sashing strips to be 2 inches wide, cut strips that are 2.5 inches wide (for seam allowances). Note the sashing width on your diagram.
Calculate how long the horizontal sashing strips will be along the length of the quilt. If your quilt blocks are all 10-inch squares and you have five quilt blocks in a row, this equals 50 inches. Next, add the width of the sashing strips that will be between each quilt block in the row. If there will be four vertical sashing strips between five quilt blocks and each sashing strip is 2 inches wide, this equals eight. Add the first number (50) to the second number (8) to reach the entire length required for the horizontal strips. You need one strip of this length between each row of quilt blocks (two strips between three rows of quilt blocks in this example).
Determine how many short vertical sashing strips you need for sewing between the vertical rows of quilt blocks. In this example of five quilt blocks in a row, you need four short vertical sashing strips for each row, which equals 12 vertical sashing strips altogether.
Divide the width of the quilting fabric (40 inches) by the width of the sashing strips (2.5 inches) to determine how many vertical sashing strips you can cut out of one strip of fabric. In this case, you will be able to cut 16 vertical strips from every 2.5-inch strip. You will only need one 2.5-inch strip to cut all the vertical sashing strips you need for your quilt (12).
Calculate how many 2.5-inch-wide 40-inch-long strips you need to make the long horizontal sashing strips. If the horizontal strips must be 58 inches long, you need more than one strip to make each horizontal sashing strip, and you will have to sew the strips together to make them longer (for 58-inch lengths). You need two 58-inch horizontal strips, so you will need three 40-inch-long strips (one strip can be cut in half and each half sewn to the other 40-inch-long strip to make just over 58 inches (cut off the excess 2 inches to make 58 inches).
Add the number of strips you need. In this example, you need one strip for the vertical sashing and three strips for the horizontal sashing to equal four strips altogether. If each strip is 2.5 inches wide, this equals 10 inches of fabric. Purchase three-eighths of a yard of fabric (13.5 inches) to give yourself a little extra fabric to work with.