Measuring a window for blinds is the only step in the entire process you cannot fix later. You can re-drill a bracket. You can shim a mount. You can return a screwdriver to the drawer. But you cannot un-cut a shade that was manufactured to your numbers.
And here is the part that gets people: the number you write down on the order form is not the number that shows up on your doorstep. The manufacturer applies their own deductions, rounds things their own way, and assumes you know a set of unwritten rules that are buried in the fine print of every order page you have ever scrolled past. So let’s walk through the real process, including the parts that exist specifically to protect the manufacturer from your mistake.
The One Rule That Overrides Everything Else: Width First, Then Height
Every blind and shade in the industry is ordered as width x height. Width first, always. If you flip them, you get a blind that fits nothing: a 36-inch-wide window with a 72-inch drop becomes a 72-inch-wide blind for a 36-inch-tall window. That is a four-figure mistake in some rooms, and there is no return policy on custom-cut anything.
The second half of the rule is equally important: measure the window, not the old blind. The blind hanging there now was made to fit whatever the last person measured, and if it was shimmed, spaced, or mounted crooked, you are just inheriting someone else’s error. Measure the opening. Ignore the existing hardware. Ignore what the previous owner did.
Grab the Right Tools (It Is a Short List)
- A steel tape measure. Not a cloth one, not the freebie from a hardware store that has been stretched by two years of use. Stretch is real and it adds up.
- A pencil and something to write on. Phone notes work, but writing on the wall trim next to each window and photographing it is faster when you have nine windows.
- A step stool. Not a chair. You are measuring a top corner with one hand.
- Skip the laser for trim work. Laser measures are fantastic for a whole room. They are terrible against molding, because the beam bounces off a bevel or catches a shadow and gives you a number that is confidently wrong.
First Fork in the Road: Inside Mount or Outside Mount
You have to decide this before you measure, because the two methods produce completely different numbers for the same window.
Inside Mount
The blind sits inside the window frame opening, recessed back toward the glass. It looks cleaner and it does not cover your trim. But it has a hard requirement: depth.
Depth is the distance from the front face of the frame to the glass. Different products need different minimums. A slim product might need as little as half an inch. Anything with a thick headrail, a cell structure, or a cassette typically wants an inch and a quarter at minimum, and two inches or more if you want it fully recessed so nothing sticks out past the wall.
If your window frame is shallow, you do not get a flush inside mount. You get brackets that protrude, or you get told the product cannot be inside mounted at all. That is not a sales trick, it is physics. Measure the depth at the top, the middle, and the bottom of the opening, because it is frequently not the same number.
Outside Mount
The blind mounts to the wall or the face of the trim, covering the window entirely. This is the correct answer for shallow frames, for windows that open outward, and for anyone who cares more about blocking light than about clean lines.
For outside mount, measure the window opening and then add overlap on every side. A common overlap is around two inches on each side and three to four inches at the top, though more is better if you have the wall space. The overlap is what kills the light leaks down the edges and around the top, which is the entire reason you are doing this.
The Actual Measuring Steps
- Measure the width at the top of the opening. Not the middle, not where it looks straightest.
- Measure the width at the middle.
- Measure the width at the bottom.
- Use the smallest of the three numbers. This is for inside mount. If you use the largest, the blind will physically not fit into the narrowest part of the opening.
- Measure the height at the left, center, and right. Use the largest number for inside mount height, because height has nowhere to fight back.
- Round down to the nearest 1/8 inch on an inside mount. Always down. Never up.
- Write it as width x height with the fractions clearly marked.
- Repeat for every single window separately. Do not measure one and apply it to the rest.
That last one is the step people skip. Two windows in the same room, built at the same time, by the same crew, in the same opening size, will routinely differ by a quarter inch. They are not the same. Measuring them the same way is how you end up with one blind that rattles and one that has to be forced.
The Stuff Nobody Bothers to Explain
- The manufacturer deducts from your number. On an inside mount, most makers subtract a small amount off the width so the product can actually move, breathe, and operate without scraping. This means the finished blind is deliberately narrower than the opening. That is correct behavior, not a defect.
- The headrail is wider than the shade. The fabric or slats may be one width while the mounting hardware at the top is a little wider. If you measure the visible slat width and order that, you will be off.
- If you measure exactly and round up, you create your own problem. Rounding up on an inside mount is the single most common way to get a product that arrives, looks right, and does not go in.
- Obstructions count as width. Window cranks, locks, latches, tilt-in mechanisms, and alarm contacts all intrude into the opening. If something sticks into the frame more than a fraction of an inch, it eats into your usable width.
- Tilt-in windows need clearance. If the sash tilts inward for cleaning, an inside mount blind can block it. Some brackets are designed to release, some are not. Know which you have before you commit.
Height: Where People Get Burned Quietly
For an inside mount, measure from the underside of the frame recess at the top down to the sill. If there is no sill, measure to the floor and note that you are doing a floor-length drop, which changes the finished look entirely.
For an outside mount, measure the window opening height and then add your top and bottom overlap. The top overlap is the important one, because that is the gap where light pours in above the frame. Mounting higher than the trim also makes the window look taller than it is, which is a free upgrade nobody charges you for.
One thing to decide early: do you want the blind to clear a handle, a sill lip, or a countertop below it? Measure that clearance before you pick a drop length. A shade that lands exactly on a sill lip will fight you for its entire life.
When the Window Refuses to Cooperate
Shallow frame, want an inside mount? Order an outside mount and stop fighting it. Alternatively, some products offer a shallow-depth bracket that lets you inside mount with a partial recess. Partial recess is fine. Nobody but you will notice.
Badly out of square? Inside mount still works if you use the smallest width and the tallest height. You will get small gaps on one side. If the gap offends you, go outside mount and let the overlap cover it.
Corner windows? Two blinds meeting at a corner will collide at the tops. Either leave a gap, order one as an outside mount that clears the other, or use a single product that turns the corner.
Wide opening, no product that big? You order multiple units and butt them together. Measure each section as its own window, and use the smallest width across the whole run for consistency, or the seams will visibly step in and out.
Read the Order Form Like It Is Trying to Trick You
Order forms will ask for width, height, and mount type, and they will quietly assume your fractions. Enter 34 1/4 as 34.25 and enter the right mount type, because plenty of systems will accept an inside mount depth that physically does not exist at your house and cheerfully manufacture the product anyway.
And understand the return policy before you type a single number. Custom-cut window coverings are made to your measurements, which means there is no resale market and no restocking. Some sellers offer a fit guarantee if you use their measuring process exactly. Read what that actually covers, because it usually does not cover you deciding you wanted a different mount type.
The Bottom Line
Measure the opening, not the old hardware. Measure every window separately, three times each direction, and take the smallest width and the largest height. Round down on inside mounts and add overlap on outside mounts. Write width first and keep your fractions visible.
Everything else in this project is recoverable. The measuring is not. Get the number right, and the rest is just drilling holes in a wall, which is the easy part.