Even if you don’t plan to make the drapes yourself, you should know how to calculate the required yardage so that you get enough fabric. A bit extra is good, not enough is not good.
A BIT of Math
Stackback is the amount of extra space past the window frame that the drapes require when open to fully expose the window. Black-out lined drapes usually require about 20% more space than just the window width.
Calculate the finished drape width with a bit of simple math. If the window is 83 inches wide and the drapes are blackout lined, you will need to add a minimum of 20% to the rod to allow the drapes to completely clear the window when open. Multiply 83 times 1.2 to find the new rod length; this will be 100 inches .
Fullness in drapery is usually calculated at 2 times or 2 1/2 times; sheers are typically 3 times. This means that the flat piece of fabric used to make the drapes will be 2 times (or 2 1/2 or 3 times )wider than the area it is meant to cover. For example, for drapes that must finish to 100 inches wide, at 2 1/2 times fullness the flat pre-pleated piece must be 250 inches wide.
Calculate the amount of fabric
Calculate the amount of fabric you need by dividing the flat, pre-pleated width by the width of the fabric. In this case, 250 divided by 54 (the typical width of drapery fabric) is 4.6, round up to 5. You will need 5 panels of fabric sewn together to create the required flat piece.
If your drapes must be 80 inches long, add 8 inches for hems and 5 inches for the top facing for the result of 93 inches. Each of your 5 pieces must be 93 inches long. Multiply 93 inches by 5 to equal 465. You need 465 inches of 54-inch wide fabric.
Divide this by 40 for the number of meters and round the figure up. In this case, the result is 12. You need 12 meters of fabric and the same amount of lining.
What if it has a pattern?
If you are using fabric with a pattern, it must be matched. No excuses, it must be matched.
Divide the panel cut length by the length of the printed pattern and round the figure up.
Use this new figure for the cut length required. For example, if the repeat is 25 inches, each piece in our example must have 4 repeats, making the cuts 100 inches long. This increases the required fabric to 13 meters.