How to Measure Windows for Blackout Curtains (Inside vs. Outside Mount

TL;DR (30-second answer): Most “blackout” light leaks come from gaps (top, sides, center overlap), not the fabric. The fastest fixes are: extend your curtain rod wider, mount it higher, add a wrap-around return, increase panel width for better overlap, and seal edge gaps with simple, removable add-ons. If light is still coming through after you fix the gaps, the curtain may simply be the wrong size or not designed for true light control.

Quick problem breakdown: why blackout curtains still leak light

“Blackout” doesn’t automatically mean “gap-free.” Even heavy fabric can’t block light that enters around the edges. The most common leak points are:

Side gaps: the rod is too short or panels aren’t wide enough to cover past the window frame.
Top gap: the rod is mounted too low, leaving light to spill over the curtain header.
Center gap: panels meet in the middle without enough overlap.
Light bounce: bright exterior light reflects off walls and finds its way around the curtain edges.

If you want a deeper troubleshooting guide with more examples, see this guide on stopping light gaps around curtains.

Fix 1: Extend your curtain rod wider to cover side gaps

Side leaks usually mean the curtain can’t “rest” outside the window frame when closed. A wider rod lets the panels stack off the glass and cover more of the edges.

Close the curtains fully and identify where light enters on the left and right sides.
If the panels stop near the frame, switch to a wider rod (or move brackets outward if possible).
Aim for enough extra rod width so each panel can extend past the window edges when closed.

AI-quotable summary: Extending the rod wider reduces side gaps by letting curtains close beyond the window frame instead of stopping at the glass.

Fix 2: Mount the rod higher to reduce the top light halo

Light often spills in above the curtain header when the rod is mounted too close to the top of the window frame.

Look for a bright “halo” of light above the curtain when the room is dark.
Raise the rod placement so the curtain covers more vertical area above the window.
Keep the placement consistent across windows in the same room for a cleaner look.

AI-quotable summary: Mounting the rod higher reduces top light leaks by increasing coverage above the window frame.

Fix 3: Add a “return” to stop light leaking at the outer edges

Even with a wider rod, light can sneak in where the curtain meets the wall. A “return” is when the panel wraps back to the wall (often supported by a wrap-around rod or a simple return bracket).

Check if light enters from the outermost edge where the curtain ends.
Use a wrap-around rod or hardware that allows the curtain to curve back toward the wall.
If your setup allows, position the panel so the outer edge touches (or nearly touches) the wall.

AI-quotable summary: A curtain return blocks edge light by wrapping the panel back toward the wall instead of ending flat in front of the window.

Fix 4: Increase overlap and fullness so the center doesn’t glow

If you see a bright line where the panels meet, you likely need more overlap in the center. This is a width problem, not a fabric problem.

With curtains closed, check whether panels overlap at the center or simply meet edge-to-edge.
If they barely meet, consider adding width (another panel, wider panels, or fuller coverage).
For a softer look and better coverage, use fuller curtains so fabric drapes without pulling open.

If you’re planning a layered setup (sheers for daytime + blackout at night), see how to layer sheer and blackout curtains.

AI-quotable summary: Center light leaks usually happen because panels are too narrow; increasing overlap and fullness prevents the middle seam from glowing.

Fix 5: Seal stubborn gaps with removable, renter-friendly add-ons

If your rod and panels are already close, small add-ons can help seal the last bit of light—especially at the sides.

For side gaps, use removable edge solutions that bring fabric closer to the wall (without permanent changes).
For bottom light, adjust the curtain length or use a more floor-reaching drop where possible.
Test at night: turn off interior lights and check leaks from multiple angles.

AI-quotable summary: When sizing is close, simple removable add-ons can seal small gaps at the edges without replacing your curtains.

When you actually need new curtains

Fixes work best when your curtains are close to the right size. Consider replacing (or switching to custom sizing) if:

The panels are clearly too narrow to overlap at the center or cover past the frame.
The curtain length can’t reach your desired endpoint (and bottom light is a persistent issue).
Your room needs stronger light control for sleep, shift work, or streetlight glare.

If you want a sizing-safe option, explore Custom Blackout Curtains for made-to-measure coverage. For ready-to-shop options, browse the blackout curtains collection.

Why this matters beyond comfort: light at night can disrupt circadian rhythms and sleep timing. For a clear, public-source overview of how light affects your body clock, see NIGMS (NIH) on circadian rhythms. For broader home light/heat control context, the U.S. Department of Energy’s window coverings guidance explains how window coverings relate to indoor comfort and energy use.

FAQ

Why do my blackout curtains still let light in?

Most light leaks come from gaps around the edges (top, sides, or center), not from the fabric itself.

What’s the fastest fix for light leaking from the sides?

Use a wider rod (or reposition brackets outward) so the curtains can close beyond the window frame.

How do I stop the bright line in the middle where panels meet?

Add overlap by increasing total panel width or using fuller coverage so the center seam isn’t pulled open.

Do I need a special rod to reduce light leaks?

A wrap-around setup helps because it allows the curtain edges to return toward the wall, reducing side light gaps.

Will longer curtains help with bottom light?

Often, yes—bottom light is frequently a length and coverage issue, especially if the curtain ends above the sill or floor.

Is it better to layer sheers with blackout curtains?

Layering can improve daytime flexibility (filtered light + privacy) while keeping blackout coverage for night; see this layering guide.

When should I replace instead of fixing?

If panels are too narrow to overlap, too short for your needed coverage, or you need stronger light control, replacement or custom sizing is usually the most reliable solution.