Most people buy blackout curtains by looking at the colour and the price. The thing that decides whether your bedroom actually goes dark is simpler: how far the curtain reaches past the window. A thick curtain that stops at the frame still lets a bright outline of light leak in around all four edges.
Light is one of the main signals your body uses to decide whether it is night. In a 2022 study from Northwestern University, people slept one night in dim light (about 3 lux) and one night with moderate room light (about 100 lux). After the brighter night, their hearts beat faster during sleep and their bodies handled sugar less well the next morning. It was a small lab study of a single night, and a street lamp outside a window is usually far dimmer than 100 lux, so this is not a reason to worry. It is a reason to see darkness as a comfort you can design, not a luxury.
If you only want softer light, or you rent and can't hang anything, a sleep mask works too. But if you want a room where you can nap at noon or sleep past a bright summer sunrise, curtains that fit properly make the biggest difference.
Enter your own measurements below. The drawing shows a warm strip wherever light would sneak around the curtain, and the numbers tell you what to look for in a pack.
Three proportions do most of the work. Rod width: the rod should reach roughly 10–15 cm past each side of the window frame, so the curtain covers the wall around the glass and light can't creep in at the edges. Rod height: mounting the rod 10–15 cm above the frame (or close to the ceiling) closes the gap at the top and makes the window look taller. Fullness: the fabric should be about 1.5 to 2 times the rod width. Flat curtains that just touch each other in the middle leave a seam of light; gathered fabric overlaps and hangs closed.
Length matters too. A curtain that nearly touches the floor stops light coming in underneath and looks calmer. Leave a centimetre or two so it doesn't drag and gather dust.
When to break the rule: if a radiator sits under the window, a floor-length curtain can trap heat behind it, so a sill-length curtain, or one that stops well above the radiator, may be the better choice. In a small room, a rod that reaches past the frame may hit a wall or a wardrobe; then mount as wide as you can and add a second layer, such as a fabric liner.
Measure before you buy, and do it with a metal tape measure rather than guessing by eye.
If you rent and don't want to drill, a tension rod fits inside a window recess, and adhesive or no-drill rod brackets exist for light curtains. Check with your landlord first. If you do drill, use wall plugs that suit your wall (brick, plasterboard and so on) and make sure a heavy, lined curtain is hung securely.
The things that matter are different at different budgets.
Even the best curtain won't be perfectly dark: a thin line of light at the edge is normal, and you can reduce it with overlap, a wider rod or a second layer. Cosy comes from a room that suits how you sleep, not from buying more.