When perfectly hung pinch-pleated curtains are drawn, the fabric lays flat between the pleats. The pleats give the curtains a large drape, which suits the scale of a large window better than the small drape of a gathered curtain. With standard pinch-pleat tape, the pleats gather almost 12.5 cm (5 inches) of fabric into a 1.2 cm (1/2 inch) space. The pleats should lie between 12.5 to 20 cm (5 to 8 inches) apart for an attractive drape.
Measure the total width of your curtain rod and the distance from the centre of the rod to the floor. If the curtain rod curves back toward the wall, include that distance as well.
Calculate the length of the curtains by adding 18.75 cm (7 1/2 inches) to the distance from the rod to the floor. This allows 1.2 cm (1/2 inch) of curtain above the rod, a 12.5 cm (5 inch) hem and 5 cm (2 inches) for folding under 2.5 cm (1 inch) at the top of the curtain and the top of the hem.
Divide the length of the curtain rod in half if you are going to have two panels on the window. Subtract 5 cm (2 inches) from this distance to allow for the extra pleat on the end and the extra fabric at each end. This fabric will not be between the pleats. The centres of the pleats should be between 12.5 to 20 cm (5 to 8 inches) apart. Chose a distance that will divide evenly into the width of your curtains. For example, if the distance is 122.5 cm (49 inches), make your pleats 17.5 cm (7 inches) apart. This means there would be seven spaces between eight pleats, with each pleat 17.5 cm (7 inches) apart.
Add 11.2 cm (4 1/2 inches) to the length measurement for each pleat. In our example, you would add 90 cm (36 inches). Also add 5 cm (2 inches) for the fabric outside the pleats, and 5 cm (2 inches) for seam allowances. For the drape we are working with, this would total 100 cm (40 inches). We would need the fabric for each panel to be 222.5 cm (89 inches) wide.
These calculations allow the panels to overlap an inch in the centre.
Buy pleater tape with the centres of the pockets 1.8 cm (3/4 inch) apart and 4-prong pinch pleat hooks. Be sure the functional part of your curtain rod extends at least one inch beyond the edge of your window on each side. This will prevent light from seeping in on the edges of the curtains.