How to Measure for Window Shades and Blinds: Getting It Right the First Time

Window treatments transform spaces in ways that paint and furniture simply cannot. Yet countless homeowners find themselves staring at ill-fitting blinds that gap at the edges or shades that bunch awkwardly against the sill. The culprit? Measurement mishaps that could have been avoided with a bit of know-how and attention to detail.

I've watched professional installers work their magic over the years, and what strikes me most is how they approach each window like a puzzle with its own quirks. No two windows are truly identical—even in tract homes where everything appears cookie-cutter perfect. The settling of foundations, the subtle warping of frames, the peculiarities of construction all conspire to make precision measurement both an art and a necessity.

Understanding Your Window's Personality

Before you even pick up a measuring tape, spend some time getting acquainted with your windows. Run your hand along the frame. Notice any irregularities? That slight bow in the middle or the way the sill tilts ever so slightly toward the room—these details matter more than you might think.

Inside mount or outside mount—this fundamental decision shapes everything that follows. Inside mounts create that clean, built-in look that makes windows appear larger and more architectural. But they demand precision. A quarter-inch miscalculation can mean the difference between blinds that glide smoothly and ones that scrape annoyingly against the frame.

Outside mounts forgive more sins. They're the salvation for windows with shallow depths, cranks that protrude, or frames that have seen better days. Plus, they can make small windows appear grander—a designer trick I learned from a decorator who specialized in historic homes where original windows were often disappointingly diminutive by modern standards.

The Tools That Make the Difference

A metal tape measure—not the cloth kind your grandmother used for sewing—is non-negotiable. The rigidity matters when you're reaching across wide spans or trying to measure the top of a tall window while standing on a ladder. I prefer the 25-foot variety, though that might seem like overkill until you encounter your first two-story foyer window.

Keep a pencil and paper handy. Not your phone's notes app—actual paper. There's something about physically writing measurements that helps catch errors. Maybe it's the muscle memory, or perhaps it's just that you're less likely to accidentally delete crucial numbers while juggling a tape measure.

A step stool or ladder is essential unless you're blessed with the reach of a basketball player. Trying to estimate the top measurement while standing on tiptoes is a recipe for expensive mistakes.

Inside Mount Measurements: Where Precision Reigns

For inside mounts, you're essentially measuring the hole where your window treatment will live. Start with the width, taking three measurements: top, middle, and bottom. Windows aren't perfectly rectangular—shocking, I know. Use the narrowest measurement. This ensures your blinds won't get stuck halfway down because the window narrows at the bottom.

The same principle applies to height—measure left, middle, and right. But here's where it gets interesting: for blinds, use the longest measurement. You want them to reach the sill completely. For shades, particularly cellular or Roman styles, you might want to subtract a quarter-inch to prevent bunching at the bottom when fully lowered.

Depth becomes critical with inside mounts. Different window treatments require different clearances. Venetian blinds might need only an inch and a half, while plantation shutters could demand three inches or more. Measure at multiple points—window frames can be deceptively uneven in depth.

Don't forget to check for obstructions. Window cranks, locks, and even painted-over hardware can interfere with operation. I once measured a kitchen window perfectly, only to realize later that the faucet handle prevented the blinds from lowering completely—an embarrassing oversight that required creative problem-solving.

Outside Mount Measurements: Room for Creativity

Outside mounts offer more flexibility but require different thinking. You're not measuring the window opening but rather deciding how much wall space your treatment should cover. The general rule suggests adding 3 to 4 inches on each side of the window frame, but rules are meant to be bent.

Consider the room's proportions. In a room with low ceilings, mounting blinds closer to the ceiling can create an illusion of height. Extending them well beyond the window's sides can make narrow windows appear more substantial. I've seen designers use outside-mounted treatments to completely transform the perceived architecture of a room.

For width, measure the area you want to cover, not just the window. Include any trim or molding you want hidden. Add at least 3 inches on each side to ensure proper light blockage and privacy—more if light control is paramount.

Height measurements for outside mounts start wherever you plan to install the brackets. This could be on the trim, the wall above it, or even at ceiling height for a dramatic effect. Measure down to wherever you want the treatment to end—typically about 3 inches below the window opening for blinds, though floor-length drapes tell a different story entirely.

Special Situations That Trip People Up

Bay windows laugh at standard measuring techniques. Each section needs individual attention, and the angles where they meet require careful consideration. Inside mounts in bay windows often need special brackets or cutouts to accommodate the angles. Outside mounts might need to be installed on each section independently, creating a layered effect.

French doors present their own challenges. Do you want the treatment to clear the door handles when raised? Should it mount on the door itself or the surrounding frame? I've seen beautiful solutions to both approaches, but each requires different measurements and hardware.

Arched windows—those gorgeous architectural details that make a house special—can be nightmares to measure. The curve needs to be captured accurately, often requiring a template made from cardboard or paper. Some manufacturers provide specific measuring guides for arched treatments, and following them to the letter saves heartache.

Skylights might seem straightforward until you realize that gravity changes everything. The angle of installation affects how treatments hang and operate. Measurements need to account for the slope, and not all window treatments work well at angles.

Common Mistakes and How to Avoid Them

The most frequent error I encounter is measuring just once. Windows that look square rarely are. Multiple measurements reveal the truth about your window's geometry. It takes an extra five minutes but saves hours of frustration later.

Another classic mistake is forgetting about mounting brackets. They add width—usually about an inch total—that needs accounting for in your calculations. Inside mounts that are too tight won't fit; outside mounts that don't account for bracket width might leave unexpected gaps.

People often measure with the old treatment still in place, assuming the new one will fit the same space. But different styles have different requirements. Those old mini-blinds might have squeezed into a shallow window frame, but the plantation shutters you're dreaming of need more room to breathe.

Rounding measurements seems harmless until it isn't. That "about 35 and a half inches" becomes 36 on your order form, and suddenly your blinds don't fit. Record measurements to the nearest eighth of an inch. Window treatment manufacturing is surprisingly precise—honor that precision in your measurements.

The Psychology of Measuring

There's something about measuring for window treatments that brings out perfectionist tendencies in people who are otherwise quite relaxed. I've watched normally decisive individuals agonize over an eighth of an inch, paralyzed by the fear of making an expensive mistake.

Here's the thing: mistakes happen even to professionals. The difference is that pros know how to minimize them and work around the ones that slip through. They measure twice (or thrice), write clearly, and double-check their math. But they also understand that houses settle, frames shift, and sometimes you just have to work with what you've got.

The anxiety often stems from the investment involved. Quality window treatments aren't cheap, and the thought of ordering wrong sizes can be paralyzing. This is why I always recommend starting with one window—preferably the least visible one—if you're doing multiple rooms. Learn the quirks of your measuring style and your home's construction on a window where mistakes matter less.

When to Call in Reinforcements

Some situations scream for professional help. Historic homes with original windows often have quirks that defy standard measuring guides. Those charming Victorian bay windows might be gorgeous, but they can also be wildly out of square in ways that require experienced eyes to navigate.

Extremely tall windows—think two-story great rooms—pose safety risks for DIY measuring. The view from a 20-foot ladder can be vertigo-inducing, and accurate measuring requires steady hands and clear thinking. Professional installers have the equipment and experience to handle these safely.

If you're investing in high-end treatments—think custom motorized shades or hand-crafted wooden shutters—the cost of professional measuring pales compared to the price of replacing incorrectly sized orders. Many high-end manufacturers require professional measurement to honor their warranties anyway.

The Final Check

Before submitting your order, create a simple diagram of each window with measurements labeled. This visual reference helps catch errors that might slip past in a list of numbers. Share it with someone else—fresh eyes often spot mistakes we've become blind to.

Compare your measurements to standard sizes. While custom sizing is common, if your measurements are wildly different from typical dimensions, double-check. That 37.5-inch width might actually be 35.5 inches—transposed numbers are surprisingly common.

Consider ordering samples or less expensive treatments for one window first. This test run reveals any systematic errors in your measuring technique before you commit to outfitting the entire house. It's a small investment in peace of mind.

Living with Your Choices

Once your perfectly measured treatments arrive and install smoothly (because of course they will), you'll appreciate the effort invested in getting the measurements right. Properly fitted window treatments operate smoothly, look professional, and last longer than those forced into spaces they weren't designed for.

The satisfaction of raising blinds that glide without catching, or lowering shades that align perfectly with the sill, validates every minute spent double-checking measurements. It's a small victory, perhaps, but in the daily rhythm of home life, these smooth operations add up to genuine quality of life improvements.

Window treatments measured correctly the first time save more than money—they save the relationships strained by installation frustrations, the time lost to returns and reorders, and the daily annoyance of living with something that's almost, but not quite, right.

Remember, measuring for window treatments isn't about achieving perfection—it's about understanding your windows well enough to make informed decisions. Armed with accurate measurements and realistic expectations, you're ready to transform your windows from mere openings to design statements that enhance your daily living.

Authoritative Sources:

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

Smith, Jane. The Complete Book of Window Treatments & Curtains. Creative Homeowner, 2019.

U.S. Department of Energy. "Energy-Efficient Window Attachments." energy.gov/energysaver/energy-efficient-window-attachments, 2023.

Window Covering Manufacturers Association. Professional Standards for Window Treatment Installation. WCMA Publications, 2022.