Measuring bay windows for treatments means measuring each window panel separately — not the overall bay width — starting with width, height, depth, and corner angles.
Bay windows look complicated to cover, but the measuring process itself is straightforward once you shift your thinking. The mistake most people make is grabbing one tape measurement across the whole bay. That number tells you almost nothing useful. Each window section of a bay — left, center, right — needs its own measurements because the angles and depths differ.
Grab a steel tape measure, a notepad, and about twenty minutes. For the best selection of blinds and shades designed for these spaces, see our roundup of top-rated bay window treatments after you finish measuring. You will need four numbers per section: width, height, depth, and the corner angle at each joint.
Measure Each Window Section: Width, Height, and Depth
Treat each bay panel as its own window. Label them left, center, and right on your notepad so the measurements stay straight.
Width: For an inside mount, measure the width at the top, middle, and bottom of each section. Use the smallest of the three numbers as your width, per the Home Depot’s measuring guide, which recommends a steel tape measure for accuracy. Openings are rarely perfectly square, and the narrowest point is what your blind must clear.
Height: Measure the height at the left, center, and right of each section. Your finished height depends on the product, but the most common approach is using the tallest measurement for a full-coverage fit. Record all three numbers so the workroom can spot an out-of-square opening.
Depth: This one gets skipped most often, and it causes the most install failures. The depth is the distance from the front of the window frame to the back, measured flat. Check it against the specific blind model’s “Inside Mount Fully Recessed Depth” on the product spec page, as Blinds.com instructs. If a handle, crank, sensor, or trim protrudes into that space, you need a shorter depth or an outside mount.
Capturing the Corner Angles of Your Bay
The angles where bay sections meet determine whether your treatments overlap cleanly or leave gaps.
For a straightforward way to capture angles without a protractor, use the cardboard template method. Take two pieces of stiff cardboard or paper. Place one flat against each window section so their edges meet in the corner. Mark where the two pieces overlap, then transfer that marked template to your notepad by tracing the angle onto paper. Your blind maker uses this tracing to match the bay geometry exactly.
This works for angled bays, box bays, and corner windows. For drapery rather than blinds, you can simplify the process: measure the three window panels (A, B, and C) plus the total depth and overall width, a method recommended by Laurel Bern’s bay-window planning guide.
Inside Mount vs. Outside Mount Measurements
The mounting style changes your numbers. Here is the difference at a glance:
| Measurement | Inside Mount | Outside Mount |
|---|---|---|
| Width | Smallest of top/middle/bottom measurements | Window width plus 3–4 inches for overlap |
| Height | Tallest of left/center/right measurements | Window height plus at least 2 inches for headrail |
| Depth | Must match product’s fully recessed depth spec | Check clearance for brackets and hardware |
| Accuracy | Nearest 1/8 inch with a steel tape | Nearest 1/8 inch with a steel tape |
Blinds.com’s guidance is the reference here: steel tape, nearest 1/8 inch, three measurement points per section. Outside mounts add 3–4 inches to width and at least 2 inches to height so the treatment covers the frame and the headrail clears any obstructions. And verify the depth is flat, not beveled, before ordering.
Common Measuring Mistakes to Avoid
These errors account for most returned blinds and shades on bay windows.
- Measuring only the overall bay width. Each section is a separate opening and needs its own numbers.
- Ignoring protrusions. Handles, cranks, sensors, and trim all eat into usable depth. Measure around them, not past them.
- Skipping the three-point method. Out-of-square openings hide at the edges. Top, middle, and bottom for width; left, center, and right for height.
- Guessing at corner angles. A paper template takes two minutes and eliminates gaps at the joints.
- Assuming one blind model fits all bays. Check each product’s recessed-depth spec against your measured depth.
References & Sources
- Home Depot. “Blinds Measurement Guide.” Details the three-point width method with a steel tape measure.
- Blinds.com. “How to Measure for a Bay Window.” Covers outside-mount additions and recessed-depth requirements.
- Bali Blinds. “Bay & Corner Window Measuring Guide.” Explains the cardboard template method for corner angles.
