A great pair of blinds can still look wrong if the measurements are off. Custom blinds are cut to size, so a small error can mean gaps, sticking, or a product that won't fit at all. The good news is that measuring correctly just takes a tape measure, a pencil, and a few minutes per window.
Here's how to get it right the first time.
Why Accurate Measuring Matters
Most blinds are made to order. That means returns are limited once a product is cut. A quarter-inch mistake can turn a beautiful custom shade into an unusable one. Careful measuring up front saves you money, time, and a second trip to the store.
Tools You'll Need
Use a steel tape measure, not a cloth one. Cloth tape can stretch and bend, which throws off your numbers. Grab a pencil, paper, and a step stool if your windows are tall. Round every measurement to the nearest eighth of an inch. That level of precision matters more than it seems.
Step 1: Choose Inside Mount or Outside Mount
Before you measure, decide where the blind will sit. Inside mount fits inside the window frame for a clean, built-in look. Outside mount attaches to the wall or trim around the window, which can hide an uneven frame and block more light. Your choice changes how you measure, so pick this first.
Measuring for Inside Mount
Inside mount takes three simple steps.
First, measure the width at the top, middle, and bottom of the window opening. Use the smallest of the three numbers. Window frames are rarely perfectly square, so this keeps your blind from being too wide to fit.
Second, measure the height at the left, center, and right sides. For horizontal blinds, use the longest measurement. For vertical blinds, use the shortest.
Third, check the depth of your window frame. Some blinds need a minimum depth to mount properly. Measure from the front of the frame to the glass at three points, and use the smallest number.
Measuring for Outside Mount
Outside mount is more forgiving, but still needs care.
Measure the width between where the left and right brackets will sit, not just the window frame itself. Then measure the height from where the top bracket will go down to the sill or your desired stopping point.
Add two to three inches of overlap on each side, so four to six inches total. This extra width blocks light gaps and gives the window a more finished look.
Common Measuring Mistakes to Avoid
A few small errors cause most measuring problems.
Measuring only once is risky. Windows shift slightly over time, so always measure at three points and use the correct minimum or maximum, depending on mount type. Guessing on depth is another common issue. Skipping this step can mean ordering a blind that physically cannot mount inside the frame. Finally, mixing up width and height measurements happens more often than you'd think, especially when ordering several windows at once. Label each measurement clearly as you go.
Quick Answers
Should I round measurements up or down? Neither. Round to the nearest eighth of an inch and report the exact number. Manufacturers build in their own adjustments for inside mount, so rounding up or down on your end can throw off the fit.
Do I need to measure every window, even if they look the same size? Yes. Windows in the same room can differ by a quarter inch or more, especially in older homes. Measure each one individually, even if they appear identical.
What if my window frame isn't square? This is common, especially in homes more than a few decades old. That's exactly why you measure width and height at three points each. Using the smallest or largest number, depending on mount type, accounts for a frame that isn't perfectly even.
When to Bring in a Professional
If you have unusual window shapes, very tall windows, or you're ordering blinds for an entire house, a professional measure removes the guesswork entirely. One error on a custom order can cost far more than the visit itself.
How CM Textiles Can Help
For 78 years, CM Textiles has helped homeowners across Montreal, Brossard, Laval, and Ottawa get their window treatments right the first time. Our team offers in-home measuring and consultations, so you never have to worry about a costly sizing mistake.
Whether you're outfitting one window or an entire home, visit us at any of our four locations or book a free measuring consultation. We'll make sure your new blinds fit perfectly, right down to the last eighth of an inch.
