Measure the opening at three points for width and height, then order to the smallest width for an inside mount and check depth so the headrail clears masonry.
Basement windows are rarely square and almost never deep, which turns a routine blind order into a return. Measure width at the top, middle, and bottom of the inside frame, and height on the left, middle, and right; treat the smallest width as your real number. Home Depot’s measuring guide and Lowe’s how-to both use this three-point method, because one middle measurement tells you nothing about a bowed frame. Use a steel tape and write numbers down. SelectBlinds advises rounding any 1/16 or 1/32 inch reading to the nearest 1/8 inch.
How Do You Measure A Basement Window Opening Correctly?
Work in three passes — width, height, depth — recording every number. For width, measure across the inside of the frame at the top, middle, and bottom, then order to the smallest reading; ordering from the largest width is the most common inside-mount failure. Measure height the same way. Sources differ on which height wins: some say order the longest, others subtract a small amount so slats rest above the sill. Bracket clearance and lift-cord access decide, so follow the product’s instructions.
Depth is the step basement owners skip, and it ends the project. Measure from the front of the frame to the glass. A shallow reveal has no room for that.
Two more checks: measure both diagonals; if they differ, the window is out of square and an inside mount will fight you. Then note every handle, lock, pipe, or lever that could block a bracket or cord.
Inside Or Outside Mount: Which Fits A Small Basement Window?
Inside mount wins on deep, square recesses; outside mount wins on shallow, uneven, or crowded ones — and most basements lean toward the second group, since block, exposed pipes, and egress hardware eat the depth an inside mount needs. Outside mount also buys light control, which matters more downstairs. Check the product before ordering. If there is no sill, or barely one, let the blind run a few inches below the sill line to close the light gap and hide the raw edge.
Where Small Basement Openings Go Wrong
Most bad orders trace to four habits: measuring once instead of at three points, ordering from the widest inside-mount reading, skipping the depth check in a shallow opening, and ignoring squareness on a shifted window. A fifth is basement-specific: not accounting for obstructions. Egress hardware, security bars, utility pipes, and a working sill all limit how the blind mounts and lifts. Measure around every protrusion, not just the frame.
Once width, height, depth, and squareness are on paper, picking a blind stops being guesswork. To skip the brand comparison, this roundup of tested basement window blinds covers options for shallow reveals and dark rooms.
| Measurement | How To Take It | Which Number To Order |
|---|---|---|
| Width | Inside frame at top, middle, bottom | Smallest reading |
| Height | Left, middle, right | Per product — longest, or minus a small allowance |
| Depth | Frame front to glass | 1–4 in. for horizontal, 3–4 in. for vertical |
| Squareness | Corner-to-corner diagonals | Must match, or switch to outside mount |
| Obstructions | Handles, locks, pipes, sill use | Note each before ordering |
| Outside-mount width | Frame plus overlap each side | About 1.5 in. per side, or 4 in. total |
| Steel tape rounding | Any 1/16 or 1/32 reading | Nearest 1/8 inch |
Ordering Once The Numbers Are Right
Round to the nearest 1/8 inch, order to the smallest inside width, and confirm depth against the product’s bracket spec before paying. Take your four numbers to the order form and let them choose the mount: out-of-square or shallow means outside mount, every time. Lowe’s blind measuring and install guide walks through the bracket clearance that trips up shallow basements.
FAQs
Do I measure the window frame or the glass?
Measure the inside of the frame, not the glass. Run the tape from left inner edge to right inner edge, and top inner edge to sill. The frame is what an inside-mount blind sits inside, so those are the manufacturer’s numbers.
What if my basement window is out of square?
If the two diagonals differ, choose an outside mount. An inside mount needs a square recess so the headrail drops in cleanly; in an uneven opening it binds or leaves gaps. Outside mount covers the gap instead of fighting it.
How much overlap do I need for an outside mount?
Basement rooms show light bleed more than any other space, so lean toward the larger overlap and confirm the figure in the product’s instructions.
References & Sources
- Home Depot. “How To Measure for Blinds and Shades” Three-point width and height measuring, depth checks, and outside-mount overlap for light control.
- Lowe’s. “How to Measure and Install Blinds” Smallest-width rule for inside mount, bracket clearance for wood and faux wood, and inside-mount depth allowances.
- SelectBlinds. “How to Measure Mini Blinds” Steel tape use and rounding 1/16 or 1/32 inch readings to the nearest 1/8 inch.
