Blackout curtains block 99% to 100% of light when installed to fully cover the window with no gaps at the sides, top, or bottom.
Throwing up any dark panel leaves streaks of light around the edges that ruin the effect. The real work of blackout curtains happens before the fabric goes up: sizing the rod wider than the frame, choosing panels with enough fullness, and hanging everything so the hem meets the floor. Get those three decisions right and the room goes genuinely dark.
Measure First: Rod Width, Panel Width, and Drop
Blackout performance lives or dies by these three numbers, so measure before you shop. Even a true blackout fabric fails if it doesn’t cover the whole opening.
Rod width: Mount the rod 4–8 inches past the window frame on each side. That extension pushes the stacked panels away from the glass, so light can’t slip past the edges. A rod that stops at the frame guarantees side leaks.
Panel width: Aim for total curtain width of about 2x to 2.5x the rod width. Two panels that just barely span the window lie flat when closed, and flat fabric lets light through the weave. Generous fullness lets the panels overlap in the middle and ripple, which seals the center gap.
Drop (length): Measure from the top of the rod (or the mounting point) down to the floor. Floor-length panels that barely kiss the floor block light best; a short hem that hovers above the sill leaves a bright strip underneath.
Install the Rod and the Panels
Blackout fabric is heavier than standard curtains, so the mounting hardware matters as much as the panels. Check that the rod and brackets are rated for the combined weight of the curtains before you put holes in the wall.
- Mark bracket positions outside the frame at your measured width. Use a level so the rod hangs straight.
- Drill and anchor properly. Drywall needs wall anchors rated for the weight; plaster and masonry need their own anchors or masonry screws. Skipping anchors is how brackets pull out at 2 a.m.
- Prep the panels. Remove packaging, shake out wrinkles, and steam or iron on low heat if needed.
- Thread the rod through the header or grommets, then set the rod on the brackets.
- Spread the panels evenly across the rod so both sides cover the frame edges, and pull the center overlap snug.
If the curtains came with tiebacks, leave them loose or skip them entirely when blocking light is the goal.
Kill the Remaining Light Leaks
Even a correctly sized install can leak from a few predictable spots. Walk the perimeter of the window and check each one.
- The top gap: Light pours over a rod mounted close to the frame. Mount the rod high — several inches above the frame, or at the ceiling — so the top of the panel seals against the wall.
- The center seam: Panels that don’t overlap in the middle leave a bright vertical line. Overlap them by a few inches.
- The rod ends: Light sneaks around the finials where the panel ends. Extending the rod further past the frame, or using panels with enough width to wrap the corners, fixes this.
- The floor: A hem that floats above the floor leaks. Floor-length or slightly puddled hems seal the bottom.
For stubborn gaps, some people add a tension rod with a second sheer or blackout liner behind the main panels. That’s the difference between “room darkening” and genuinely dark rooms.
Room Darkening vs. True Blackout
Shopping gets confusing because the labels aren’t interchangeable. Retailer guides make the distinction clear: “true blackout” panels block 99% to 100% of incoming light, while “room darkening” panels block roughly 60% to 85%. If you’re a shift worker or a light sleeper, read the product description for the percentage claim before you buy — don’t assume the word “blackout” on the package means total darkness.
Fabric weight and weave drive the difference. True blackout curtains use a dense, often triple-weave or coated fabric that physically stops light. Room darkening panels filter most light but let a glow through, which suits living rooms or nurseries where total blackout feels too cave-like. If you need flexibility, a true blackout panel layered over a lighter curtain gives you both options on the same rod.
If you’re ready to shop, our tested roundup of the best extra-long blackout curtains covers floor-length panels that actually seal the bottom gap.
One final check after hanging: stand back and look for any daylight between the panel edge and the wall. If you see a sliver, the fix is almost always a wider rod, not a different curtain. The JCPenney curtains and drapes buying guide confirms the same sizing logic: measure wide, mount high, and let the panels reach the floor.
FAQs
Do blackout curtains need to touch the floor?
For maximum light blocking, yes. A hem that stops above the floor leaves a visible strip of light underneath. Floor-length panels that gently kiss the ground seal that gap, which is why most blackout installations aim for the floor or a slight puddle.
What’s the best way to hang blackout curtains to block light?
Mount the rod high and wide. Extend it at least 4–8 inches past the window frame on each side so panels can cover the wall beyond the glass, and set it several inches above the frame — or at the ceiling — to stop top leaks. Use panels roughly 2 to 2.5 times the rod width.
Will a room darkening curtain work as well as blackout?
No. Room darkening panels block about 60% to 85% of light, while true blackout curtains block 99% to 100% when installed correctly. For bedrooms, nurseries, or shift work, look for the percentage claim on the label rather than assuming the word “blackout” guarantees total darkness.
References & Sources
- JCPenney. “Curtains & Drapes Guide.” Retailer guide covering rod width, panel fullness, and drop measurements.
