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How curtain fabric is estimated
Curtains are made from vertical panels of fabric. First, the calculator works out how many panels are needed by multiplying the window width by a fullness factor — the extra width that creates gathers and folds — then dividing by the fabric width. The number of panels is rounded up to the next whole panel.
Each panel's length is the window height plus a hem and header allowance (for the bottom hem, top header, and rod casing). The total is summed and converted from inches to yards (divide by 36).
Fullness controls the look: 2.0× is a standard full gather, 2.5× gives a more luxurious, deeply pleated look, and 1.5× is a lighter, more tailored drape. Fabric width is usually 54 inches for decorator/upholstery fabric, though some sheers and specialty bolts come 44–60 in or wider — check your bolt.
For floor-length curtains, measure from the rod to the floor (or to the sill for a sill-length look) and add a few extra inches if you want a puddle effect.
Rod width and mounting tips
Most designers mount the rod wider than the window so the open curtains stack clear of the glass — a good rule of thumb is 6–12 extra inches on each side. Because that added width also needs fullness, measure the rod, not just the window, when you want the curtains to close fully. If you're using a patterned fabric, buy extra to match the repeat across panels, and always check whether your fabric is sold by the yard with a fixed bolt width.
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