Most blackout curtains do their job: the cloth itself is thick or coated enough to stop nearly all the light that hits it. Yet plenty of people hang a pair and still lie there looking at a bright line down each side. That light is not coming through the curtain. It is coming around it.
So the real question is not "how dark is the fabric?" but "how well does the curtain cover the window?" Three gaps cause nearly all the leaks: the sides, the top and the bottom. Try them below. Move the sliders and watch where the light creeps in.
Light travels in straight lines, but it also bounces off the wall, the window frame and the sill. A curtain that ends exactly at the frame leaves a thin gap at each side where light slides past the fabric. Curtains also hang a few centimetres in front of the wall, and a bright window makes even a narrow gap look like a glowing line in a dark room.
The fix is overlap. A common rule of thumb is to let the curtain extend about 10–15 cm past the window frame on each side, so the gap sits behind the fabric and the light has nowhere to go. At the top, a rod set 10–15 cm above the frame (or even closer to the ceiling) stops light spilling over the heading. At the bottom, let the hem fall at least past the sill, and to the floor if you can: it looks calmer and hides the glow from below.
A pair of curtains also needs to meet in the middle. If the two panels are only just as wide as the rail, they pull apart as the cloth drapes into folds. Curtains look and block better when they are cut wider than the rail, usually around 1.5 to 2 times its length, so the folds can overlap at the centre.
Measure the window frame, not the glass: the width from outside edge to outside edge, and the height from the top of the frame to the sill. Then decide where the rod will go. Sit a tape measure against the wall to check how high you can mount it: ceiling height, a curtain box or a beam can limit you. Finally, measure from the planned rod position down to the floor (or sill) and add a little for the rings or hooks. The planner below turns your frame width into a rail length and a fabric width.
| What | Result |
|---|---|
| Rail or rod length (about 12 cm each side) | |
| Total curtain width, gentle folds (1.5×) | |
| Total curtain width, full folds (2×) |
Many sleep specialists agree that a dark room supports sleep, and that light from outside, such as street lamps, headlights and early sun, can get in the way. Light inside the room counts too: a bright standby light or phone screen can bother you as much as a gap in the curtains. Before buying anything, check what actually wakes you. If it is a thin line from one side, a wider rod may fix it for free.
You don't have to replace what you have. A few cheaper steps often work: move the rod higher and wider, add a blackout lining behind curtains you already own, or look for second-hand panels in a heavier fabric. A sleep mask costs very little and travels with you. And if you rent, a tension rod or a rail with no drilling can still give you the width you need.
One safety note: heavy blackout curtains are heavier than they look. Make sure the rod and its fixings are fitted into something solid, and keep cords for any blinds out of reach of small children.
At a low budget, look for a clearly marked blackout fabric, a long enough drop for your measured height, and a width you can double up at the centre. Mid-range curtains often add a thicker weave or a lining that also takes the chill off a cold window. At the top end, you are mainly paying for extra length and width and a heavier hang, which suits tall windows and rooms with a high ceiling. Whatever the price, the sizes matter more than the label: measure first, then buy.