Curtain Size Calculator

Curtain sizing is a geometry problem with two independent axes: width, governed by the fullness ratio, and drop, governed by mounting height plus finishing allowances. The rod width, not the glass width, is the true baseline; an outside mount adds a side extension of roughly 15 to 20 cm per side so the stacked panels clear the aperture and light does not leak around the edges. Multiplying rod width by a fullness ratio gives the total flat width of cloth that must be gathered onto the rod. Fullness of 2.0 is the working default for pinch pleat and pencil pleat headings, 1.5 suits flat or tab top panels, and sheers usually need 2.5 to 3.0 because an open weave looks sparse when only lightly gathered. The drop calculation runs the other way: take the finished hanging length, then add the cloth consumed by construction. A double fold bottom hem of 5 to 7.5 cm needs 10 to 15 cm of extra length, and headers range from about 5 cm for a rod pocket to 10 to 12 cm for pinch pleats. Fabric is then bought in bolt widths, commonly 137 cm (54 inches) for decorator cloth, so the required flat width is divided by that figure and rounded up to whole widths, which is why total yardage jumps in steps. The main limitations are that the arithmetic ignores vertical pattern repeat, which can add most of one repeat per drop, and ignores shrinkage; linens can lose 3 to 5 percent on first wash.

Cloth hung at openings predates glazing, but the measured, calculated curtain belongs to the industrial era. Before the twentieth century, drapery width was set in the workroom by eye and by the number of loom widths a household could afford, and gathering was done with hand stitched running threads. The change came through hardware and tape. In 1907 Charles W. Kirsch of Sturgis, Michigan produced the flat curtain rod, and the Kirsch company went on to popularize the traverse rod that let heavy drapes stack and slide, which made rod width rather than window width the natural unit of measurement. The second change was woven heading tape. From the mid 1930s the Manchester firm Rufflette manufactured curtain tape with drawcords and pockets that converted a flat panel into an evenly pleated one, and each tape carried a recommended fullness ratio printed by the maker. That standardization is why modern instructions speak in multipliers such as 1.5x, 2x and 2.5x rather than in yards of a specific cloth. Postwar mass housing with repeatable window openings, together with the spread of 54 inch decorator bolts once wider looms became common, completed the shift. By the 1960s pattern books and department store workrooms were publishing the same width times fullness, plus hem and header, arithmetic used today.