So this is how i did my math.
Box pleat curtain tutorial.
My window is 46 wide.
Once you have cut your fabric lay the ride side of your drapery fabric down on top of your lining.
As a starting place for the amount of fabric needed for the pleats you can multiply the distance you want the curtain panel to cover by about 2 or 3 depending on how full you want the curtains.
6 30 it should say lining width face fabric width 6 inches instead of 3 inches.
3 5 3 5 4 11.
Return leading edge 4 for sewing the sides.
My board was 42 wide.
For my curtain i only wanted 1 pleat.
If you are mounting your curtain on a 1 4 board you need to add 5 for the ends of the board and 8 for the pleat in the middle.
For each pleat you need to add 8 inches of fabric.
She does a great job describing how to measure how much fabric you will need with the addition of the pleats hems etc.
We factored in 8 per pleat an additional 3 on each side for the depth of the board to wrap around the return or the side of the board 1 2 for the 1 4 hem on each side and about a 1 2 for each end to wrap around the back of the board equaling 65 5 total in length.
We decided on 2 pleats so 3 sections of flat fabric.
Press all the pleats flat with an iron creating a 3 inch pleat so that the seam is behind the exact center of the pleat.
I altered her tutorial just a bit and made my own box pleat curtain.