Most blackout curtains that disappoint were not bad curtains. They were the wrong width, hung from the wrong height, with a gap of light down each side. Five minutes with a tape measure fixes all three.
1. Width: measure the rod, not the glass
Measure the full width of the window including the frame. Your rod should extend 4 to 8 inches past the frame on each side, so the closed curtain overlaps the wall and light cannot slip in at the edges. For a full, gathered look, the total width of your panels should be 1.5 to 2 times the rod width. Two 52-inch panels (104 inches together) cover a rod up to about 70 inches comfortably.
2. Length: decide where the hem lands first
Measure from the top of the rod down to the point you want the hem to reach. Three common choices: the sill (a clean, short look for kitchens), 4 inches below the sill (the most forgiving), or ½ inch above the floor (the most light-tight). Panels that hang through a rod pocket sit with their top edge at the rod, so the measurement from the rod down is the length you need.
3. Height: hang higher than you think
Mount the rod 4 to 6 inches above the window frame, or halfway to the ceiling in a low room. Higher rods stop light leaking over the top of the curtain and make the window look taller. This is the step most people skip and the reason most rooms are not as dark as the packaging promised.
Quick reference
- Rod width = window width + 8 to 16 inches
- Panel width total = 1.5 to 2 × rod width
- 84-inch panels: floor-length with the rod mounted just above a standard window frame
- 96-inch panels: floor-length with the rod mounted higher, closer to the ceiling
- 108-inch panels: tall windows and high ceilings
If you are between two lengths, take the longer one — a hem that puddles slightly is fine; a hem that stops short lets light in underneath.