How this calculator works
Curtain width is the window width multiplied by a fullness ratio, because flat fabric must be wider than the window to fold. Two panels are then sized from the total. The drop depends on where the curtain finishes: at the sill it is the window height, at the apron it adds about 15 cm, and at the floor it adds about 30 cm below the sill. Café curtains cover half the window, and extra-long curtains add about 45 cm. These are starting points; the exact amount depends on how far the rod sits above the window and how far the sill projects.
Formula
total curtain width = window width x fullness ratio. width per panel = total width / number of panels. drop = window height x style factor + style allowance.
Worked example
A window 150 cm wide and 140 cm high with deluxe fullness needs 300 cm of curtain width. In two panels that is 150 cm each. An apron-length drop adds 15 cm, giving 155 cm, while a floor-length drop adds 30 cm, giving 170 cm.
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Frequently asked questions
Should curtains be wider than the window?
Yes. The fabric needs to gather, and the panels should also cover the wall beside the window when closed so light does not leak around the edges. A ratio of 1.5 to 2 over the rod length is the usual range.
Where should the rod go?
Above the window frame and wider than the opening. Raising the rod makes the window look taller; extending it 15 to 30 cm on each side lets the open curtains sit clear of the glass. Measure the rod position, not just the window, when ordering.
How do I measure for floor-length curtains?
Measure from the rod or track to the floor, then subtract 1 to 2 cm so the hem just clears the floor. If you want a puddle, add 5 to 10 cm. The window-height figure in this calculator is not enough on its own for that measurement.
Do curtains need lining?
Lining improves insulation, protects the fabric from sunlight and makes the curtains hang better. It does not change the outer dimensions, but it adds fabric and work, so budget for a second length roughly equal to the face fabric.