How to Choose the Right Curtain Size for Your Window
Curtains cut to the exact window width hang flat and leak light at the sides. Measure width from the rod, not the glass, then multiply by a fullness ratio to get the fabric width. Rods are usually mounted a few inches past each side of the frame (guides commonly suggest about 3β6 inches) so open panels stack off the glass.
Fullness Is a Choice, Not a Standard
Fullness is a drapery-workroom convention, not a code or regulation. Many guides treat about 2Γ the rod width as standard, 1.5Γ as light gathers, and 2.5β3Γ for very full looks or sheers. Heavy fabrics such as velvet usually need less; light fabrics need more. That is why this calculator lets you pick 1.5Γ, 2.0Γ or 2.5Γ rather than fixing one number.
How to Calculate Curtain Length
Length is measured from the rod. For a sill-length curtain, choose how far below the frame it should fall. For floor-length, enter the height from the floor to the bottom of the frame and choose a clearance β a small gap keeps the hem from dragging, while 0 cm just kisses the floor. For the most accurate result, measure from the bottom of the hook or ring.
Blackout Panels and Shrinkage
For blackout curtains, mounting the rod higher and wider than the frame reduces light leaking around the edges. Cotton and linen may shrink when washed, so follow the care label and allow extra length if the maker recommends it.
This calculator is for reference only. Fullness, rod extension and floor clearance are user-selected conventions, not official standards.
Frequently Asked Questions
Measure the rod, not the window. Many workroom guides use about 2Γ the rod width as standard, 1.5Γ for light gathers and 2.5Γ or more for full looks or sheers. It is a convention, so choose by fabric weight.
It depends on the retailer. Some list flat fabric width and others list finished (gathered) width, so check before comparing it with your calculated fabric width or rod length.