Fabric for gathered curtain panels at your chosen fullness.
Adds ~16\u2033 to length for header and hem and ~10% to width for side hems. Estimate.
Gathered curtains must be wider than the window or the rod, since the extra fabric forms the folds. The total width of fabric needed is the finished width to cover multiplied by the fullness ratio, and the number of panels is that width divided by the width of the fabric. A fullness of 2 to 2.5 is typical for gathered or pleated curtains.
Lightweight and sheer fabrics look better with more fullness, and heavy fabrics with a little less. The length of each panel is the finished drop plus allowances for the hem at the bottom and the heading at the top, which can add several inches.
Measure the rod and decide where the curtains will hang before you buy. Mounting the rod higher and wider than the window makes the room look bigger, and curtains that just clear the floor or rest a little on it look the most polished.
Patterned fabrics need extra for matching, and lined curtains need extra fabric for the lining. Pre wash fabrics that may shrink, and hang the curtains for a few days before you hem them so the fabric can settle and the hem comes out level.