For blackout curtains that really make the room dark, choose a total curtain width of 1.5 to 2 times the width of the window, and a pole that extends 15 to 20 cm beyond each side of the frame. For the drop, the curtain starts 10 to 15 cm above the window and falls to the floor (within 1 or 2 cm) or at least 15 to 20 cm below the sill. It is mostly this overlap, in width and height, that stops light leaking round the sides.
In short
- Width: 1.5 to 2 times the window width, all panels together.
- Overhang: 15 to 20 cm each side, and 10 to 15 cm above.
- Drop: to the floor, or below the sill.
- What to measure: the pole (or where it will go), not the glass.
Why size matters more for blackout curtains
Blackout fabric stops the light that tries to pass through it. It can do nothing about the light that goes around it. In a bedroom, the light leaking round a curtain that is too narrow or too short is enough to brighten the room at sunrise. Morning light matters: the Sleep Foundation points out that light, even dim light, can disturb sleep and the body clock. With an ordinary curtain, an approximate size will do. With a blackout curtain, a few extra centimetres of overlap make all the difference.
How to measure your window for curtains
- Measure the width of the frame, from one outer edge to the other (not just the glass).
- Add the overhang: 15 to 20 cm each side. Example: a 120 cm window needs a 150 to 160 cm pole.
- Set the pole height: 10 to 15 cm above the frame, more if the ceiling allows.
- Measure the drop from the top of the pole (or the rings) to the floor, or to 15 to 20 cm below the sill.
These are trade conventions, not a standard. They are there to save you from the curtain that is just too small, the most common mistake.
How wide should curtains be for a window?
A curtain is not hung flat: it forms folds. So you need more fabric than the width of the pole.
| Pole width | Recommended total curtain width | In practice |
|---|
| 100 cm | 150 to 200 cm | 1 wide panel or 2 panels of 75 to 100 cm |
| 150 cm | 225 to 300 cm | 2 panels of about 140 cm |
| 200 cm | 300 to 400 cm | 2 to 3 panels of 140 cm |
| 280 cm (patio door) | 420 to 560 cm | 3 to 4 panels of 140 cm |
The ratio depends on the heading. With eyelets, 1.5 to 2 times the width gives nice waves. A wave or pleated curtain often follows the manufacturer's guidance.
What drop for blackout curtains?
- Floor length: the darkest option. The curtain stops 1 or 2 cm above the floor so it does not drag, or just brushes it if you like that look.
- Below the sill: if there is a radiator under the window, a long curtain would trap the heat. Stop it 15 to 20 cm below the sill, and add a blind in the recess if you want full darkness.
- Standard drops: in continental Europe, ready-made curtains are often 240, 250 or 260 cm long. Measure before buying: under a low ceiling you will need a hem.
One panel or two?
Two panels meeting in the middle often leave a line of light. Let them overlap by 5 to 10 cm, or close them with a clip or magnet. A single wide panel avoids the problem on a small window. On a patio door, three or four panels are easier to handle.
Curtains for a roof window or a French door
A roof window cannot take an ordinary curtain: it would hang down into the room. It needs a blind made for the window, or a blackout film. For a French door, measure as for a window, but make the pole wide enough for the open curtains to clear the doorway: 25 to 30 cm of overhang on the opening side.
Curtains too short or too narrow: what can you do?
Three fixes: lower the pole a few centimetres if the frame allows, add a band of blackout fabric at the bottom, or add a blind in the recess. For curtains that are too narrow, the simplest is to add an extra panel in the same colour rather than stretching the two you have.
Curtain or blind?
A curtain covers the window and its edges: it blocks light and keeps in a little warmth in winter. A blind sits in the recess or on the frame, more discreet, but light sometimes gets in at the sides. The two combine very well; we look at day and night blinds in day and night blinds: do they block light?. Not everyone needs total darkness, but if you work nights or the street is lit, blackout becomes precious.
To finish
For a bedroom that stays dark at daybreak, choose the size before the model. Our thermal blackout curtain with eyelets hangs on a standard pole, and the wave blackout curtain falls in regular folds. The dimensions of each are on its product page.
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