Getting the right fit for mini blinds isn’t just about making your window look sharp—it’s about avoiding that annoying sliver of light that sneaks in on the sides or the blinds that hang so loosely they clatter every time a breeze passes. Nobody wants to install a brand new set of blinds only to discover they’re too wide to slide into the frame or so narrow they leave a gap that ruins the room’s coziness. The good news is that measuring for mini blinds is a straightforward task you can handle with nothing more than a steel tape measure, a pencil, and a bit of patience. And once you get the hang of it, you’ll feel like a pro, saving the cost of a return shipment and the frustration of a second trip to the hardware store.

The first decision you have to make is whether you want the blinds to sit inside the window frame or outside it. Inside mount means the blinds are tucked within the window opening, giving a clean, built-in look. Outside mount means the blinds are installed on the wall or trim above the window, covering the entire opening and then some. Inside mount is more common for mini blinds because it keeps the window sill usable and feels more streamlined. But you need the window frame to be deep enough—usually at least an inch and a half—to accommodate the headrail. If your frame is shallow or your window isn’t perfectly square, outside mount can be the better choice because it hides imperfections.

For an inside mount, you need to measure the width of the window opening in three different places: the top, the middle, and the bottom. Run your tape measure across the opening from the left edge of the frame to the right edge, making sure the tape is flat and not twisting. Write down each measurement to the nearest eighth of an inch. More often than not, those three numbers won’t be identical—old houses settle, new houses shift, and wood frames expand with humidity. Do not use the widest measurement; instead, take the smallest of the three. That way, the blinds will slide in easily without you having to force them. Then, do the same for the height. Measure from the top of the frame down to the window sill at the left, the center, and the right. For height, you’ll want the longest measurement, because you want the blinds to reach the sill without leaving a gap at the bottom.

Now you might be thinking, “But what if the window sill is sloped or the inside of the frame has a weird lip?” Good question. If your sill slopes inward, you’ll still take the height from the top of the frame down to the highest point of the sill. If there’s a small molding or a stop inside the frame that the blinds have to clear, make sure the headrail you order is designed to fit past that obstruction. Many mini blinds come with an option to trim the slats, but the headrail is fixed, so that measurement has to be spot on.

For an outside mount, you have a little more freedom. You want the blinds to overlap the window opening on all sides so no light sneaks around the edges. Measure the width of the entire window casing or the area you want to cover, and then add at least three inches on each side for good coverage—so six inches total added to the width. For the height, measure from where you plan to hang the bracket to the point you want the blinds to end. Usually that’s at the top of the window trim to either the bottom of the sill or a few inches below it. Again, add extra—about three inches above and below is standard. The beauty of outside mount is that you don’t have to worry about a slightly crooked frame because the blinds will cover the whole show.

One mistake even seasoned homeowners make is using a cloth tape measure. Those are great for sewing curtains, but they stretch over time and can give you a wobbly reading. Always use a metal tape measure that locks in place. And be sure to record every measurement as you go because there is nothing more frustrating than realizing you’ve forgotten the middle width and have to pull out the tape again. Write them down in plain inches, not fractions like 1/2 or 3/4—you can convert when you order, but it helps to keep raw numbers.

What about windows that have obstructions, like cranks on casement windows or a sink that sticks out in front? Measure the depth of the window frame from front to back. If you’re doing an inside mount, you need enough clearance for the blind’s headrail and the tilt mechanism. For mini blinds, typical headrail depth is about half an inch to an inch, but if you have a window crank that projects into the opening, you might need a cutout in the blind or you’ll have to switch to an outside mount. Similarly, if you plan to use cordless mini blinds—which are a fantastic choice if you have little kids or pets—the headrail is slightly thicker to hold the spring mechanism, so check the manufacturer’s depth requirement.

Finally, when you’re ready to order, remember that many manufacturers will ask you to order the exact width measurement for an inside mount. They usually make the blind a quarter to a half inch narrower than the opening to give you that smooth fit. If you’re measuring for an outside mount, you simply give them the width and height you want, and they build it to size. Some stores offer free returns on custom sizes, but it’s better to get it right the first time.

Take your time with the tape measure, double check each number, and you’ll have mini blinds that look like they were made for your windows—because they were. Once they’re hung, you’ll wonder why you ever put up with those old, warbed vinyl blinds that never quite covered the sunlight. And when a friend compliments how crisp and clean your windows look, you can smile and say, “I measured them myself.”