If you measure a corner window like one big opening, you can end up with blinds that hit each other at the corner. I treat it as two separate windows, then I plan which blind runs full and which one butts into it with a small deduction for the headrail.
Here’s the short version:
- I measure each side on its own
- I record every number in inches to the nearest 1/8 inch
- For inside mounts, I check depth first, then measure width, height, and corner clearance
- For outside mounts, I measure the coverage area, not just the frame
- I use the smallest height for inside mounts
- If width varies by more than 1/4 inch, the opening may be out of square
- I check for handles, cranks, locks, trim, and uneven walls before ordering
The main point: one blind usually goes all the way to the corner, and the other stops short so the two headrails don’t collide.
A fast way to think about it:
| Mount type | What I measure first | Main thing to watch |
|---|---|---|
| Inside mount | Frame depth | Headrail clearance at the corner |
| Outside mount | Total coverage area | Overlap and meeting point |
About 1/8 inch can change the fit, and even a 1/4 inch out-of-square difference can cause trouble. So I keep the process simple, write down each number, and double-check the corner seam before I order.
Below, I’ll walk through the measuring steps in plain English.
How to Measure Corner Windows for Blinds: Step-by-Step Guide
How to measure for Single Roller Blinds – Corner window, inside mount
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Tools and Prep Before You Measure
Before you start, grab a tape measure, pencil, and a worksheet. The point here is simple: make sure both blinds can fit neatly into the corner without rubbing or getting stuck. It also helps to check the frame depth and spot anything that might get in the way before you write down numbers.
Check Depth and Obstructions First
Measure the depth of each window frame at the top of the opening to see if an inside mount will work. While you’re there, look for anything that could block the blind, such as window cranks, handles, locks, deep trim, tile, or uneven walls [1].
Record Measurements in a Clear Format
Label each window clearly: Window A (Full) for one side of the corner and Window B (Butt side) for the other. This makes it much easier to track both sides of the same corner opening without mixing them up.
Use a worksheet with separate rows for width, height, depth, mount type, and any deductions.
| Measurement Field | Window A (Full) | Window B (Butt side) | Notes |
|---|---|---|---|
| Width (W) | Measure to the inside corner | ||
| Height (H) | Use the smallest of the 3 measurements | ||
| Depth (D) | Check for flush mount | ||
| Mount Type | Inside / Outside | Inside / Outside | |
| Deduction/clearance | N/A (usually 0) | Headrail projection | Based on product type |
Measure width and height in three places so you can catch openings that are a little out of square. Use the smallest height. After that, you’re ready to measure each side for the inside mount.
How to Measure Inside-Mount Corner Windows
With the window labels done and the depth check out of the way, measure each corner opening on its own. Corner frames are rarely perfectly square, so you can’t assume both sides will behave the same way. For inside-mount blinds, the main job is simple: leave enough room so both blinds meet neatly at the corner.
Measure Widths and Plan the Full-and-Butt Layout
Once you have both opening widths, choose which blind will be the full blind and which will be the butt blind. Put the full blind on the larger window or on the side where privacy matters more; the butt blind stops short of the full blind’s headrail [2][3].
A handy trick is to use a cardboard depth template cut to the headrail depth. Set it into the corner, then measure from the template’s edge to the opposite frame [1]. That gives you a more exact read than eyeballing the overlap.
Measure Heights and Check for Out-of-Square Openings
Measure the height at the left, center, and right side of each window. Then use the smallest number as your order height [1][4].
For width, watch for uneven openings. If the width measurements differ by more than 1/4 inch, the opening is out of square and may need an adjustment before you order [1][4]. That small mismatch can turn into a fit problem fast.
Account for Light Gaps and Privacy
After you size both blinds, take one more look at the corner seam. This is where privacy and clearance come into play. The full-and-butt layout helps cut down the light gap at the corner, but the deduction on the butt blind needs to be exact so the blinds don’t rub [2][3].
You’re aiming for a clean meeting point in the corner, with smooth operation and as little light spill as possible.
How to Measure Outside-Mount Corner Windows
If an inside mount won’t work because the frame is too shallow, uneven, or out of square, go with an outside-mount plan. It gives you more control over fit and helps the blinds sit cleanly across the corner.
Measure the Total Coverage Area
Start by marking where you want the finished top, side, and bottom edges to land. Then measure each window from those lines, not just from the frame itself.
Add 1.5 to 3 inches of overlap on each side for better light control and privacy at the corner seam [1]. For height, measure from your planned top mounting line down to the bottom edge you want. If the two sides come out different, use the larger height.
Plan How the Two Blinds Meet at the Corner
Outside-mount corner blinds use a meeting-point layout. One blind runs all the way to the corner, and the other stops at its edge. This setup helps cut down on light leaking through the corner.
Pick the longer window, or the side that needs more privacy, as the through blind. If the hardware sticks out, leave a small gap so the headrails don’t bump into each other [1][3].
Before you order, write down each detail on the order sheet or schedule a free measurement:
- Which blind is the through blind
- Which blind is the butt blind
- The overlap amount
- Any gap needed at the corner
- Any required corner deduction
Common Measuring Mistakes to Avoid
Before you order, check for the corner-window mistakes that trip people up most often.
With corner windows, even a small measuring slip can turn into a problem once the blinds show up. One of the biggest issues is using a flexible tape measure. Use a rigid steel tape instead so your corner measurement stays steady and exact. Measure both sides on their own too, because corner openings often look the same at a glance but don’t match perfectly.
Another common miss is skipping the minimum flush-mount depth check or forgetting to account for headrail clearance at the corner. If the headrails bump into each other, the blinds won’t open or close the way they should. Hardware matters too. Handles, cranks, and locks can get in the way of the blind’s travel path, especially in tight or odd-shaped corners. Before you submit the order, do one last pass against your order sheet.
What to Check Before You Order
For both Window A and Window B, confirm each of these points:
- Mount type confirmed – inside or outside, based on the depth you actually measured
- Width deductions applied – the butt blind width adjusted for the product’s depth deduction
- Height recorded correctly – for inside mounts, use the smallest measurement
- Corner clearance verified – make sure headrails won’t hit each other or any hardware
- Obstructions noted – watch for handles, cranks, or locks that change blind placement
Depth rules can also change by product type, so double-check them before ordering. Roller shades, solar shades, woven wood shades, and shutters each come with their own minimum depth specs. A setup that fits one product may not fit another.
FAQs
Which side should be the full blind?
One blind should be the full blind, also called the through blind. The other should be the butt blind. In most cases, put the full blind on the longer window.
The through blind mounts all the way into the corner, which lets it pass in front of the butt blind. It’s also a smart pick for the main window, or for the one where you want more privacy, because it helps cut down the gap between the blinds.
How do I measure corner deduction accurately?
When measuring corner windows, do not make your own corner deductions.
Instead, give the manufacturer the exact window opening measurements. They’ll handle the deductions needed for a proper fit. That matters because corner blinds need to meet cleanly, and even a small miscalculation can throw things off.
For inside-mount corner blinds, one blind is usually the bypass (full) blind, which extends into the corner. The other is the butt blind, which stops against it.
If you’d rather not measure on your own, AAA Blind & Shutter Factory can also help with professional measuring.
What if my corner windows are out of square?
Measure both diagonals of the window frame from corner to corner. This shows how far out of square the opening is.
If those two measurements differ by more than 1/2 inch, the window is usually too far out of square for an inside mount.
In that case, an outside mount is the better pick. The blinds hang outside the window opening, which helps cover imperfections that can lead to gaps or make the blinds look crooked.
