How to Measure Windows for Blinds: The Art of Getting It Right the First Time
I've measured hundreds of windows over the years, and I still remember the sinking feeling when my first set of custom blinds arrived two inches too narrow. That expensive mistake taught me something crucial: measuring windows isn't just about running a tape measure across the frame. It's about understanding the subtle dance between your window's quirks and the blind system you're planning to install.
Most people think measuring windows is straightforward—width times height, done. But windows, especially in older homes, have personalities. They lean, they bow, they pretend to be square when they're actually parallelograms in disguise. And if you're not paying attention, they'll humble you real quick when those blinds show up.
The Philosophy of Inside vs. Outside Mount
Before you even touch that tape measure, you need to make a fundamental decision that will affect everything else: are you going inside or outside the window frame?
Inside mount blinds sit within the window frame itself, creating that clean, built-in look that makes interior designers swoon. They're like a well-tailored suit—when they fit right, nothing else compares. But here's the thing: inside mounts are unforgiving. Your window frame needs at least 1½ inches of flat mounting surface for most blinds, though some slimmer options can work with less. I once spent an afternoon with a client whose 1920s craftsman had beautiful, ornate window frames with exactly 1¼ inches of mounting depth. We ended up going outside mount, and honestly? It looked fantastic.
Outside mount blinds attach to the wall or molding around your window. They're the forgiving friend who doesn't judge your window's imperfections. Got a window that's out of square? Outside mount covers those sins. Need to block more light? Outside mount lets you extend beyond the window opening. The trade-off is that they project further into your room and have a more pronounced presence.
The Inside Mount Deep Dive
When measuring for inside mount blinds, precision isn't just important—it's everything. Start by checking your window frame depth. Stick your tape measure straight back from the front edge of the frame. Different blind styles need different depths: cellular shades might only need an inch, while wood blinds with decorative tapes could require three inches or more.
Now for the actual measurements. Here's where most people mess up: they measure once and call it good. Your window isn't uniform. Measure the width at the top, middle, and bottom of your window opening. Write down all three numbers. Do the same for height—left side, center, and right side.
For width, you want the narrowest measurement. Yes, the narrowest. The blind manufacturer will make precise deductions (usually about ⅜ inch total) to ensure your blinds fit without scraping. I learned this the hard way when I ordered based on the widest measurement, thinking I was being smart. The blinds wedged in so tight I needed a rubber mallet to install them.
For height, use the longest measurement. You want your blinds to fully cover the window opening without gaps at the bottom. A blind that's slightly too long is far better than one that's too short—trust me on this.
The Outside Mount Strategy
Outside mount measuring feels more relaxed, but it has its own considerations. You're not constrained by the window frame, which means you get to decide how much coverage you want. This is where personal preference meets practical necessity.
For width, I typically recommend adding 3 to 4 inches on each side of the window opening. This overlap ensures privacy and light control while creating visual weight that balances the window architecturally. Some folks go wider—I've seen dramatic floor-to-ceiling installations that extend 8 inches on each side, turning a modest window into a focal point.
Height is where you can really play. Mounting your blinds 4 to 6 inches above the window frame creates an illusion of taller windows—a trick I picked up from a staging professional who transformed dumpy ranch windows into something almost stately. Just make sure you have enough flat mounting surface wherever you plan to install the brackets.
The Reality of Imperfect Windows
Let me tell you about the time I measured a window in a 1960s split-level that was a full inch wider at the bottom than the top. The homeowner hadn't noticed in thirty years of living there, but once I pointed it out, she couldn't unsee it. This is more common than you'd think, especially in older homes where settling has worked its magic.
For inside mounts in out-of-square windows, always defer to the smallest measurements. The gaps created by irregular openings can sometimes be masked with a valance or cornice, but blinds that don't fit simply don't fit.
Outside mounts are your friend when dealing with wonky windows. By extending beyond the frame, you create new, straight lines that override the window's irregularities. I've saved many a renovation budget by suggesting outside mount blinds instead of replacing architecturally challenged windows.
Special Situations That'll Test Your Patience
French doors require a different approach entirely. You need clearance for the door handles, which usually means outside mount unless you have unusually deep-set handles. Measure from the outer edge of the handle to determine your minimum projection. Add at least ¼ inch for safety—nothing's worse than blinds that catch every time you open the door.
Bay windows are their own beast. Each section needs individual blinds, and the angles where they meet can be tricky. I measure each section as its own window, but I also check the clearance at the corners. Blinds that collide when raised defeat the whole purpose of having operational window treatments.
Corner windows without a post between them? That's advanced-level stuff. You'll need to decide which blind takes priority at the corner, as they can't both extend to the edge. Usually, I favor the window that's more prominent from the room's main viewing angle.
The Measuring Tools That Matter
A metal tape measure is non-negotiable. Those fabric ones from the sewing kit won't cut it—they stretch, they sag, and they lie. Get a 25-foot metal tape measure with a wide blade that won't buckle when extended.
Write everything down immediately. I use a simple notebook where I sketch the window and note all measurements around the drawing. Don't trust your memory, especially if you're measuring multiple windows. Label everything obsessively: "Master bedroom, north wall, left window" is better than "bedroom 1."
A step ladder is worth its weight in gold. Trying to measure the top of a window while standing on your tiptoes on a chair is a recipe for inaccurate measurements and possibly a trip to the emergency room.
The Ordering Moment of Truth
When you're ready to order, double-check everything. Then check again. Most custom blinds are non-returnable because they're cut to your specifications. That sick feeling when you realize you've transposed two numbers? It's expensive.
If you're ordering multiple blinds, consider ordering one first as a test, especially if you're unsure about inside versus outside mount or if your windows have quirks. Yes, you might pay more in shipping, but it's cheaper than replacing multiple incorrect blinds.
Some manufacturers offer a measuring guarantee—if their blinds don't fit based on the measurements you provided, they'll remake them. Read the fine print carefully. These guarantees often require professional installation and documentation of your original measurements.
Final Thoughts from the Field
After all these years, I still get a little nervous when those boxes arrive. Did I measure correctly? Will they fit? That moment when you hold the blind up to the window for the first time never gets old.
The truth is, measuring windows for blinds is both simpler and more complex than most people realize. Simple because it's just numbers on a tape measure. Complex because those numbers have to account for the reality of your specific windows, with all their character and quirks.
Take your time. Measure twice, order once. And remember—even professionals occasionally order the wrong size. The difference is we've learned to measure our way out of most problems before they happen. Now you can too.
Authoritative Sources:
Hunter Douglas. Window Treatment Measuring and Installation Guide. Hunter Douglas Window Fashions, 2019.
Kirsch, Sarah. The Complete Guide to Window Treatments. Creative Homeowner, 2018.
Smith, Michael S. Elements of Style: Designing a Home and a Life. Rizzoli, 2020.
U.S. Department of Energy. "Energy-Efficient Window Attachments." Energy.gov, Office of Energy Efficiency & Renewable Energy, 2021.
Window Covering Manufacturers Association. Professional Standards for Window Covering Installation. WCMA Publications, 2020.