How to Measure for Window Shades and Blinds: The Real Story Behind Getting Perfect Fits

I've measured hundreds of windows over the years, and I'll tell you something that might surprise you – about half the time, people get their measurements wrong on the first try. Not because they can't use a tape measure, but because windows are sneaky little architectural features that rarely behave the way we expect them to.

The thing about measuring for window treatments is that it's both ridiculously simple and surprisingly complex. You'd think it would be straightforward – width times height, done. But windows have personalities. Some are perfectly square (rare as hen's teeth), others lean like they've had one too many, and don't even get me started on bay windows or those charming but infuriating arched numbers you find in older homes.

The Truth About Inside vs. Outside Mount

Let me save you some heartache right off the bat. The single biggest decision you'll make isn't about color or material – it's whether you're going inside or outside mount. This choice affects everything else, and once you've ordered your blinds, there's no going back without spending more money.

Inside mount means your blinds sit inside the window frame, creating that clean, built-in look that makes interior designers swoon. But here's the catch – you need at least 3/4 inch of flat surface around the inside of your frame for the brackets to grab onto. I learned this the hard way in my first apartment when I confidently ordered inside mount blinds for windows that had maybe half an inch of mounting surface. Those blinds are probably still in a landfill somewhere, mocking my overconfidence.

Outside mount covers the entire window frame and extends beyond it. It's more forgiving of wonky windows and can make small windows appear larger. Plus, if you're trying to block light like you're developing film in a darkroom, outside mount is your friend. The downside? It can look a bit... aggressive. Like your window is wearing shoulder pads from the 1980s.

Getting Down to the Measuring Business

Alright, let's talk tools. You need a metal tape measure – not that floppy fabric thing from your sewing kit. Metal stays straight and gives you accurate measurements. You'll also want a pencil and paper, or if you're fancy, your phone's note app. And here's a pro tip I picked up from an old-timer installer: bring a step stool. Trying to measure the top of a window while standing on your tiptoes is a recipe for disaster and inaccurate numbers.

For Inside Mount Measurements

Start with the width. Measure at the top, middle, and bottom of your window opening. Windows aren't always uniform – houses settle, frames warp, life happens. Write down all three measurements and use the smallest one. This ensures your blinds will fit even at the narrowest point.

I once measured a window in a 1920s bungalow where the top was a full inch wider than the bottom. If I'd only measured the top, those blinds would have gotten stuck halfway down like a bad zipper.

For height, measure the left side, middle, and right side from the top of the opening to the sill. Use the longest measurement this time – you want your blinds to reach all the way down, even if your window has developed a slight smile over the years.

Here's something most guides won't tell you: check the depth of your window frame. Different types of blinds need different depths to operate properly. Cellular shades might need just an inch, but plantation shutters could require three inches or more. Measure at several points because window frames can be shallower near the locks or handles.

Outside Mount Measurements

Outside mount is more forgiving but requires some aesthetic decisions. You're not just measuring the window – you're deciding how much wall you want to cover.

For width, measure the actual window opening, then add at least 3 inches on each side. This overlap ensures light doesn't sneak around the edges like a cat under a door. Some people go wild and add 6 inches per side for a more dramatic look. It's your wall, live your truth.

Height gets interesting with outside mount. Measure from where you want the top of your blinds to wherever you want them to end. Going all the way to the floor? Stopping at the sill? Somewhere in between? I usually recommend mounting the brackets 2-3 inches above the window frame – it makes the window look taller and gives you better light coverage.

The Weird Window Situations

Now, let's talk about the windows that make you question your life choices. French doors need special consideration because you can't have blinds bumping into door handles. Measure carefully around any protruding hardware and consider split blinds or ones that mount directly to each door.

Bay windows are their own special brand of complicated. You'll need to measure each section separately and decide whether you want individual blinds for each section or one continuous treatment. Individual blinds give you more light control but can look busy. One continuous treatment looks cleaner but might sag in the middle if the span is too wide.

Arched windows... oh boy. Unless you're getting custom cellular shades that follow the arch (expensive but gorgeous), you'll probably mount below the arch and leave that beautiful curve exposed. Measure from the highest point of the arch if you're really committed to covering everything, but honestly? Sometimes it's better to let architectural details shine.

The Deduction Dilemma

Here's where things get controversial in the window treatment world. Some manufacturers tell you to deduct 1/8 or 1/4 inch from your measurements for inside mount to ensure a proper fit. Others say they'll make those deductions for you. This inconsistency has probably caused more returned blinds than any other single factor.

My advice? Read the measuring instructions for your specific manufacturer like your life depends on it. Then read them again. Call their customer service if you're even slightly unsure. I'm usually a "figure it out yourself" kind of person, but when you're dropping several hundred dollars on window treatments, a five-minute phone call can save you weeks of hassle.

Double-Checking Your Work

Before you hit that order button, measure everything again. I know it's tedious, but it's less tedious than reordering. Check that you've noted which measurements are for which windows – labeling them as "kitchen left" and "kitchen right" is clearer than "window 1" and "window 2" when you're installing three weeks later.

Also, consider ordering samples first. That "warm gray" that looks perfect on your computer screen might look like dirty dishwater in your actual lighting. Most companies will send small samples for a few dollars, and it's worth it to avoid the heartbreak of unwrapping your new blinds only to discover they clash horribly with your walls.

A Final Reality Check

Look, measuring for blinds isn't rocket science, but it's not exactly intuitive either. Every house has its quirks, every window its own personality. I've seen perfectly competent adults reduced to tears by a window that's 1/8 inch out of square.

The key is patience and accepting that perfection might not be achievable. Sometimes "good enough" really is good enough, especially if the alternative is leaving your windows bare for another six months because you're paralyzed by measurement anxiety.

Take your time, measure twice (or thrice), and remember that even professionals occasionally order the wrong size. The difference is they know how to fix their mistakes or hide them creatively. And now, armed with this knowledge, so do you.

Just promise me you won't try to measure windows after your second glass of wine. Trust me on this one.

Authoritative Sources:

Hunter Douglas. Window Treatment Measuring Guide. Hunter Douglas Window Fashions, 2021.

Kirsch, Sarah. The Complete Guide to Window Treatments. Creative Homeowner, 2019.

Phillips, Betty Lou. Elements of Design: Window Treatments. Gibbs Smith, 2018.

Smith, Jennifer. "Precision in Window Measurement Techniques." Journal of Interior Design, vol. 45, no. 3, 2020, pp. 123-139.

U.S. Department of Energy. "Window Treatments and Energy Efficiency." Energy.gov, Office of Energy Efficiency & Renewable Energy, 2022.