How to choose blackout curtains that actually block light

Blackout curtains should do exactly what the name suggests. In practice, a lot of people buy curtains labelled as blackout, hang them, and still find light creeping in at the edges, the top, or through the fabric itself. This guide explains what actually makes a curtain block light effectively, the most common reasons blackout curtains underperform, and how to get the results you are looking for.

What makes a curtain truly blackout?

The word "blackout" refers to the lining, not the face fabric. A blackout curtain is a standard curtain with a specialist lining sewn to the back of the fabric, or sometimes bonded directly into it. This lining is typically a tightly woven fabric with a white or cream coated backing designed to reflect light rather than let it pass through.

The quality of that lining varies significantly between products. A true blackout lining will block 99% or more of incoming light. A lower-quality version, sometimes called dim-out or light-reducing lining, reduces light without eliminating it. Both types are sold under the word "blackout" in some markets, which is where the confusion comes from.

When assessing a blackout curtain, the lining matters more than the face fabric colour or thickness. A heavy dark fabric with a poor lining will let in more light than a lighter fabric with a proper blackout backing.

Our blackout curtains range includes options across several heading types and fabric styles, all with a proper light-blocking lining. Our thermal curtains combine a blackout lining with a thermal treatment, which keeps rooms cooler in summer and warmer in winter alongside blocking light. If you are primarily trying to keep a room cool as well as dark, the thermal range is worth considering, our guide on how to sleep in the summer heat covers this in more detail.

What is the difference between blackout and dim-out curtains?

Blackout curtains block the vast majority of light passing through the fabric, typically 95 to 99% depending on the lining quality. In a well-fitted room with good blackout curtains, you should not be able to read in daylight with the curtains closed.

Dim-out curtains (also sold as light-reducing or room-darkening curtains) reduce the amount of light entering the room without eliminating it. They are useful for filtering bright sunlight in a living room or making a bedroom darker for daytime naps, but they will not create the kind of darkness needed for shift workers, babies, or anyone who genuinely cannot sleep with any light present.

If you need a room to be genuinely dark, check the lining type rather than just the product name. A curtain marketed as "blackout" should specify whether it blocks 90%, 95%, 99%, or 100% of light, or indicate that it carries a blackout lining rather than just a light-reducing one.

Why do blackout curtains sometimes fail to block light?

The fabric is only one part of the equation. Most blackout curtain failures come down to fitting rather than the curtain itself.

The curtains are too narrow. This is the most common problem. If the curtains only cover the window frame exactly, light will pour in around the sides the moment there is any gap. Blackout curtains need to extend well beyond the window frame on each side, at least 15 to 20 centimetres, and should overlap each other in the middle by at least 10 centimetres when closed.

The pole is too close to the window. When a curtain pole sits tight against the window frame or wall, light can travel behind the curtain and bounce into the room even when the curtain itself blocks it from front to back. Mounting the pole closer to the ceiling (rather than just above the window) and using a pole with returns (the curved ends that bring the pole back to the wall) eliminates most of the gap at the sides. Our curtain poles range includes poles with returns for exactly this purpose.

The curtains are too short. Light comes in at the bottom if the curtain stops short of the floor or leaves a gap below the sill. For proper blackout, curtains should reach the floor. If floor-length curtains are not practical, they should at least extend a full 15 centimetres below the windowsill.

The heading type creates gaps. Eyelet curtains have metal rings through which the pole threads. The rings themselves can create small gaps at the top of the curtain where light enters around the rod. Pencil pleat curtains, hung on a pole with returns, give a tighter seal. For a bedroom or nursery where light blocking is a priority, pencil pleat is the better heading choice. Our guide to eyelet vs pencil pleat curtains covers this in more detail.

The curtains are hung too far from the ceiling. Every centimetre of gap between the top of the curtain and the ceiling is a potential path for light. Hanging the pole as high as possible, ideally within a few centimetres of the ceiling, minimises the area for light to travel around.

How to get a proper blackout fit

The following steps address the most common fitting failures:

Width. Buy curtains wide enough to extend 15 to 20 centimetres beyond the window frame on each side. When closed, the two curtain panels should overlap each other in the middle by at least 10 centimetres. For pencil pleat curtains, allow 2 to 2.5 times the total pole width in fabric. For eyelet, 1.5 to 2 times.

Height. Mount the pole as close to the ceiling as is practical. The curtains should reach the floor or to at least 15 centimetres below the sill.

Pole returns. Fit a pole with returns so the pole curves back to the wall at each end. This closes off the gap at the sides of the curtain and prevents light from travelling behind the fabric.

Overlap at the centre. Check that the two curtain panels overlap sufficiently when closed. A gap in the middle, however small, will be noticeable in a dark room.

For detailed measuring guidance, see our how to measure for curtains guide.

Which rooms benefit most from blackout curtains?

Bedrooms. The most common use. Even a small amount of light in a bedroom can disrupt sleep quality, particularly in summer when mornings are bright early. For anyone who is sensitive to light when sleeping, a properly fitted blackout curtain makes a significant practical difference.

Nurseries and children's rooms. Blackout curtains in a nursery help babies and young children sleep at predictable times regardless of the hour or season. The earlier and later hours of summer daylight, when it is still light outside at bedtime and light again before most children need to wake, are where this matters most.

Shift workers. Anyone sleeping during the day needs a room that is genuinely dark, not just dimmed. This is the use case where a proper blackout lining and careful fitting matters most, since daytime light levels are far higher than the ambient glow that might be acceptable overnight.

Home cinema rooms. For a dedicated viewing room or a living room used for film watching, blackout curtains on the windows most likely to catch direct sun during viewing hours make a real difference to screen visibility.

South and west-facing rooms. These rooms catch the strongest and longest direct sunlight. In summer, a south-facing bedroom can become uncomfortably warm by mid-morning. Thermal blackout curtains, kept closed during the hottest part of the day, reduce both the light and the heat entering the room.

Blackout curtains and thermal performance

A number of blackout curtains also carry a thermal lining or treatment, which means they do more than block light. The thermal coating reflects heat in both directions: outward in summer to keep the room cooler, and inward in winter to reduce heat loss through the window. For most UK bedrooms, a thermal blackout curtain is a practical all-round choice.

Our thermal curtains range combines both properties. The Luna range within this collection is available in eyelet and pencil pleat heading types and has been designed specifically for rooms where both light control and temperature management matter.

Frequently asked questions

Do blackout curtains completely block light? A well-made blackout curtain with a proper lining will block 95 to 99% of light through the fabric. However, light can still enter around the sides, top, and bottom if the curtains are not fitted correctly. The curtain fabric is only one part of the solution, fitting matters as much as the product itself.

What is the difference between blackout and dim-out curtains? Blackout curtains block nearly all light passing through the fabric, typically 95% or more. Dim-out or light-reducing curtains filter and reduce light without eliminating it. Both are sometimes marketed using the word "blackout", so checking the specific lining type and light-blocking percentage is worth doing before buying.

Do eyelet blackout curtains work as well as pencil pleat? Pencil pleat blackout curtains on a pole with returns tend to give a tighter seal, since the heading fits more closely against the wall and ceiling with fewer gaps. Eyelet curtains have metal rings which can create small gaps at the top. For the highest level of light blocking, pencil pleat is the better heading choice. For most bedrooms, good eyelet blackout curtains fitted correctly will still be effective.

How do I stop light coming in at the sides of blackout curtains? Mount the pole with returns so it curves back to the wall at each end. Make sure the curtains extend at least 15 to 20 centimetres beyond the window frame on each side, and overlap in the middle by at least 10 centimetres when closed.

How high should I hang blackout curtains? As close to the ceiling as possible. The gap between the top of the curtain and the ceiling is a potential path for light, so minimising this gap makes the curtains more effective. The curtains themselves should reach the floor.

Are thermal curtains the same as blackout curtains? Not necessarily, but many thermal curtains include a blackout lining. Thermal curtains are primarily designed to regulate temperature (keeping heat out in summer and in during winter). Blackout curtains are primarily designed to block light. A curtain with both a thermal and blackout lining does both.

Can I make normal curtains blackout? Yes. Blackout lining can be bought separately and either sewn to the back of existing curtains or clipped to a separate lining track behind them. This is a lower-cost option if you already have curtains you like but want to improve their light-blocking performance.

Browse our blackout curtains and thermal curtains ranges, or see our guide on eyelet vs pencil pleat curtains if you are still deciding on heading type. For measuring guidance, our how to measure for curtains guide covers everything you need to get the fit right.

This guide is put together by our curtains team, who advise on and stock the full range of heading types, linings, and curtain hardware we carry.