The base formula is: window width × fullness ratio ÷ fabric width = number of widths needed. Fullness is typically 1.5 for flat panels up to 3 for very full gathers.
Multiply the number of widths by the curtain drop (plus 25 cm for top and bottom hems) to get the total meterage.
If the fabric has a pattern repeat, add one repeat per width so the design lines up across all panels.
One last tip: 300 cm fabrics can be railroaded, removing the horizontal seam entirely — in that case the calculation is reversed.
Tags:Technical