Gathered curtains need more fabric than the glass. Fullness of 2.0 means twice the window width in finished fabric. Divide that by the fabric width (often 54 inches) and round up to whole widths (drops).
Each drop is cut longer than the finished length: header (rod pocket or pleat) plus hem. Eight inches header and eight inches hem is a common starting point.
For a pair of panels, this tool bumps an odd width count up by one so each side gets the same number of widths.
Paper on the same wall is the wallpaper calculator. If you only have a rough opening, get the area from the square foot calculator.
How this calculation works
widths = ceil(window × fullness ÷ fabric width). Yards = widths × (drop + header + hem) ÷ 36, then round up to 1/8 yard.