About this tool
The Curtain Size Calculator is a free, in-browser tool that turns a window's dimensions into the curtain panel count, per-panel width, finished length, and total fabric yardage. Getting these right is what separates flat, skimpy curtains from full, gathered drapery. Enter the window width and height, the rod's mount-to-floor height, a fullness factor, the rod extension, and the panel width, choose a hanging style, and it returns everything you need to buy or sew panels. All math runs locally in your browser.
Width is driven by fullness. The tool first widens the coverage by the rod extension on each side (total rod width = window width + 2 × extension), then multiplies by the fullness factor — 1.5 for a subtle look, 2.0 for the recommended standard gather, or 2.5–3.0 for luxurious fullness. Dividing that total fabric width by the flat panel width and rounding up to an even number gives the panel count for a symmetric pair. Length depends on style: sill and apron lengths are measured from the window (clearance above the window plus its height, apron adding 4 in), while floor length is the rod height minus 0.5 in and puddle adds about 4 in of pooling fabric. Each panel's cut length adds 16 in for the top header and bottom hem (8 in each), rounded up to the pattern repeat if one is entered, and total yardage = cut length × panels ÷ 36.
As a worked example, a 36-inch window with 8 inches of rod extension per side and a fullness of 2.0 needs (36 + 16) × 2 = 104 inches of fabric width — about three 50-inch panels, rounded up to an even four for a balanced pair. For a floor style on an 84-inch rod, the finished length is 83.5 in and the cut length is 99.5 in per panel, giving roughly 11 yards for four panels before any pattern repeat.