If you have ever stood in your bedroom at six in the morning, glaring at the sliver of sunlight sneaking through the edges of your blinds, you already know the appeal of blackout shades. They promise a room so dark you could mistake noon for midnight. They help kids nap longer, shift workers sleep soundly, and movie nights feel like a real theater. But there is a common frustration that homeowners run into after spending good money on new blackout shades: light still gets in around the sides. The culprit is almost never the shade material itself. The culprit is almost always the measurement.

Getting the right measurement for blackout shades is a different animal than measuring for standard blinds. With regular blinds, a little extra light around the edges is usually no big deal. With blackout shades, even a quarter-inch gap can turn a pitch-black room into one with unwanted stripes of daylight. The good news is that with a careful approach and a steady hand, you can measure once and get it right.

The first big decision you have to make is whether you want an inside mount or an outside mount. An inside mount means the shade fits inside the window frame. This gives a clean, built-in look and shows off your window trim. An outside mount means the shade is installed above and beyond the frame, covering the entire window opening and a little bit of the wall around it. For blackout shades, outside mount is almost always the better choice. Because the shade extends past the window frame on all sides, there are no gaps for light to sneak through. If you are serious about blocking light and you have the wall space to spare, go with an outside mount.

If you prefer the look of an inside mount or you have windows with shallow frames, you can still make blackout shades work, but you need to be extremely precise. Start with a metal tape measure. Cloth tapes can stretch over time and lead to errors. Measure the width of the window opening at the top, in the middle, and at the bottom. Write down the smallest number. Window frames are rarely perfectly square, and using the smallest width ensures your shade will fit without scraping against the sides. Do the same for the height: measure at the left, the center, and the right, and use the longest height measurement. This is because you want the shade to be long enough to reach the sill, even if the frame slopes a little.

One mistake that catches many homeowners is failing to account for the top gap. Even with an inside mount, there is often a small space between the headrail of the shade and the top of the window frame. Light pours in through this gap. Many manufacturers now offer blackout shade systems with special channels or fabric wraps that seal this area, but you have to know about it before you order. If your chosen shade does not include a top seal, you might need to add a separate piece of trim or a foam strip to block that light.

For outside mount shades, the rules are simpler but just as important. You want the shade to overlap the window opening on all sides. A good rule of thumb is to add at least three inches to the width on each side, for a total of six inches wider than the window. For height, you usually want to mount above the window trim and let the shade drop down past the sill. The exact overlap depends on your window and your wall space, but the principle is the same: if you can see the window frame from the side, light can get in. Cover the opening completely.

A final tip that makes a noticeable difference is to measure the depth of your window frame. Some blackout shades have a cassette or a valance that sticks out from the wall. If you have deep window trim, the shade might not sit flat against the wall, leaving a gap at the top. Checking depth before you buy saves you the headache of returning a shade that does not mount flush.

Measuring for blackout shades does not require any special skills. It just requires a little patience and the understanding that light finds the tiniest openings. Take your time. Measure in three spots. Write everything down. And when in doubt, choose the outside mount for the most reliable seal. Your future self, sleeping in on a Saturday morning with no idea the sun is even up, will thank you.