If you have ever stood in the aisle of a home improvement store staring at rows of blinds and thinking, “I have no idea what size to buy,“ you are not alone. Measuring for blinds seems like it should be straightforward, but one small mistake can leave you with a gap on the side or a blind that drags on the sill. The good news is that with a tape measure, a pencil, and about ten minutes of your time, you can get perfect measurements every single time. Let’s break down the process in a way that makes sense, whether you are a seasoned DIYer or someone who has never held a tape measure before.

First, you need to decide what kind of mount you want. This is the single most important decision because it changes how you measure everything else. An inside mount means the blinds sit inside the window frame, tucked neatly between the jambs. This looks clean and modern, and it lets the window trim stay fully visible. An outside mount means the blinds are attached to the wall or molding above the window, covering the entire opening, and often extending a few inches beyond each side. Inside mounts are great for windows with deep frames and square corners. Outside mounts are your best friend if your window frame is shallow, uneven, or if you want to make a small window look bigger by adding width.

For an inside mount, you are measuring the exact width and height of the opening where the blind will live. Take your tape measure and measure the width at the top, the middle, and the bottom of the window opening. Write down all three numbers. Why three? Because windows are rarely perfect rectangles. The frame might bow slightly or have settled over time. You want to use the narrowest width of the three as your final measurement, because the blind has to fit inside that smallest gap without rubbing against the sides. Then measure the height in the same way, from the top of the opening down to the sill, on the left, the center, and the right. Again, use the shortest height measurement. When you order an inside mount blind, you will often be asked for the exact opening size, and the manufacturer makes the blind slightly narrower so it slides in smoothly. You do not need to subtract anything yourself, just report what you measured.

For an outside mount, the process is actually easier because you have more freedom. You get to decide how much overlap you want. The general rule of thumb is to add about two inches to the width of the window frame, meaning one extra inch on each side, to block out as much light as possible. And for the height, you want the blind to extend about one inch beyond the top of the frame, and then come down to either the sill or a few inches below it. That measurement is entirely up to you, depending on whether you want the blind to sit above a radiator or just reach the bottom of the trim. The key is to measure the width from the outside edges of where you want the blind to cover, not the window frame itself. You are essentially measuring the total width of the area you want the blind to hide. And for height, measure from the top of the mounting point all the way down to where you want the blind to end.

Now, let’s talk about the little details that people often forget. First, always use a metal tape measure, not a cloth one, because cloth stretches and gives you a false reading. Second, write everything down in inches and eighths of an inch. Do not round up to the nearest quarter or half inch. A blind that is a quarter inch too wide will rub and bind, and one that is a quarter inch too narrow will let light stream in from the sides. Third, check that your tape measure is held flat and level. If you are measuring an inside mount and you hold the tape at a slight angle, even a five degree tilt can add an eighth of an inch or more to your number. So always measure straight across and straight down.

Another common mistake is forgetting to account for hardware that sticks into the window opening, like the little latches on a double hung window, or a crank handle on a casement window. If you are doing an inside mount, look inside the frame to see if there are any protruding pieces that would interfere with the blind. If there are, you might need to switch to an outside mount or order a blind that has a special cutout. It is better to catch that before you order than to try to return a custom blind later.

One last tip: measure every window separately, even if they look identical. You would be amazed at how much difference a house can settle, or how much a carpenter’s slight mistake from twenty years ago can show up now. What looks like a matching pair of windows in the living room can easily differ by a quarter inch in real life. So measure each window as its own project, and label your measurements clearly, like “living room left” and “living room right.“

After you have your measurements, ordering blinds becomes a breeze. Most online retailers will ask for the mount type and the exact dimensions you wrote down. If you are buying from a store, bring those numbers with you, and ask a salesperson to double check your work if you are unsure. A little time spent carefully measuring now will save you from the headache of exchanging the wrong size, drilling extra holes, or living with gaps that annoy you every day. And honestly, once you have done it the first time, you will wonder why you ever thought it was hard. So grab that tape measure, go stand in front of your window, and take the first step toward a beautiful, perfectly fitted set of blinds.