Window Blind Size Calculator
Learn how to measure windows for blinds and get the exact size to order. This inside mount blind size calculator applies labeled factory deductions, the outside mount blinds size calculator adds adjustable overlap, and the mount chooser tells you which mount your window can take.
Inside mount blind size
Measure the window recess (narrowest width; shortest height for a safe fit), then pick a preset below. To measure a window for blinds inside mount, use a steel tape and record to the nearest 1/8 inch. The order width inside mount blinds require is the recess width minus the deduction; blind height measurement goes to the sill (or minus 1/4 inch for sill clearance).
Deduction sources: Wayfair 1/4–1/2 in; SelectBlinds up to 3/4 in by collection; Bali 1/8–1/4 in light gap per side. "Factory deducts" (order exact) is what Lowe's, Home Depot and SelectBlinds recommend — never subtract twice.
Outside mount blind size
Decide how much overlap for outside mount blinds you want, then measure the window opening. The outside mount blinds width overlap per side is added twice (left + right) to the window width; height gets a top extension plus a bottom extension down to the sill or floor.
Overlap sources: SelectBlinds Canada recommends at least 2 in per side plus above and below; Lowe's vertical-blinds guide uses 4 in per side and at least 3 in top/bottom. No deduction is ever taken on an outside mount.
Mount chooser: can this window take an inside mount?
Enter your window recess depth — how deep should a window be for blinds? Measure from the front edge of the frame to the glass at three spots and use the shallowest. This checks the window recess depth for inside mount blinds against the minimum window depth for blinds published by manufacturers.
Depth guidance (labeled, not a standard): 2.5 in+ → inside mount fits flush for most blind types (faux wood recessed 2.5 in; vertical blinds 3.5 in); 1–2.5 in → inside mount fits many products but the headrail may protrude — check the product spec; below 1 in → outside mount (hard floor 3/4 in per The Shade Store).
Window blind measuring guide
This window blind size guide covers the full window blind measuring guide workflow in four steps:
- Pick a mount. Inside mount sits in the recess (clean, shows trim); outside mount covers the frame (better light blocking, makes windows look larger). Unsure? Use the mount chooser above.
- Measure width × height. Width: top, middle, bottom — keep the narrowest. Height: left, center, right — keep the shortest for a guaranteed fit. Record width first, then height, to the nearest 1/8 inch.
- Measure the depth. Front of frame to glass, three spots, keep the shallowest. Then corner to corner both ways — that is how to check window square for blinds: if the diagonals differ by more than ~1/2 inch, prefer an outside mount.
- Apply mount math. Inside: order exact (factory deducts) or subtract 1/4–3/4 in yourself — never both. Outside: add your overlap. Re-measure every window, even ones that look identical.
Looking for a standard blind sizes chart? Stock sizes vary by retailer and most blinds are cut to order — this calculator gives you the exact order size, which works for custom and stock alike.
Boundary: this tool sizes blinds and shades that mount in or over the window recess. Sizing fabric panels that hang from a rod is a different job — use our curtain size calculator for panel width, rod length and hanging height.
Sources: Wayfair measuring guides, Lowe's custom-blinds guide, Home Depot faux-wood manual, SelectBlinds measuring instructions, The Shade Store. See SOURCES in the project notes.
Frequently asked questions
What size blinds for a 36 inch window?
For a 36-inch window opening with an inside mount, order a blind width of 35.5 (35 1/2″) using the standard 1/2-inch deduction, and order the height exact at 60 inches if the blind will rest on the sill. That gives you the blind size for the window opening: 35.5 wide by 60 tall. If your retailer applies its own factory deduction (Lowe's, Home Depot and SelectBlinds all do), order the exact 36-inch recess width instead and let the factory subtract.
Do blinds need to be smaller than the window for an inside mount?
Yes. The blind width deduction for an inside mount is the factory's operating clearance: typically 1/4 to 1/2 inch total (SelectBlinds deducts up to 3/4 inch on some wood collections). Report your exact recess width and let the factory deduct. For a 48-inch recess the finished blind comes out between 47.5 (47 1/2″) and 47.75 (47 3/4″) inches wide. Never subtract first and then let the factory subtract again: that double deduction turns 48 inches into 47 (47″) inches of blind, leaving big light gaps.
How much overlap do I need for outside mount blinds?
A good rule of thumb is 2 inches of overlap per side. A 36-inch window with 2-inch side overlap needs a 40-inch blind (40″). For height, add about 3 inches above and 3 inches below: a 48-inch window becomes a 54-inch blind (54″). For vertical blinds, Lowe's recommends 4 inches per side, so a 60-inch opening needs a 68-inch blind (68″). More overlap blocks more light; the calculator above lets you adjust every side.
Inside mount vs outside mount blinds — which should I choose?
Measure your window recess depth first. At 2.5 inches or deeper, inside mount blinds fit flush for most blind types. Between 1 and 2.5 inches, an inside mount usually still fits but the headrail may sit slightly proud of the wall — check the product's minimum depth spec. Below 1 inch, choose an outside mount. Beyond depth: inside mounts show off your trim and look built-in, while outside mounts hide light gaps better and make the window look larger.
How deep should a window be for inside mount blinds?
It depends on the blind type (compiled manufacturer minimums): aluminum mini blinds need 3/4 inch to secure and 1.5 inches to sit fully recessed; faux wood 1.5 / 2.5 inches; roller shades 1 / 1.5 inches; cellular shades 1.5 / 2 inches; vertical blinds 2.5 / 3.5 inches. The Shade Store sets a hard floor: below 3/4 inch of depth, outside mount is your only option. Use the mount chooser tab above with your measured depth.
How do I measure a window for blinds — and how do I check it is square?
To measure a window for blinds inside mount, measure the width at the top, middle and bottom and keep the narrowest — for example 36, 35.5 and 35.75 inches means you use 35.5 inches. Measure the height at the left, center and right and use the shortest for a guaranteed fit. Then measure the diagonal from corner to corner both ways: if the two diagonals differ by more than about 1/2 inch, the frame is out of square and an outside mount will hide it better (Wayfair measuring guide).
How to measure for blinds in cm?
Measure in centimeters exactly as you would in inches, then apply the same deductions: a 90 cm recess with the standard 1/2-inch (1.27 cm) deduction orders at 88.73 cm wide. A 150 cm height with the 1/4-inch (0.64 cm) sill clearance orders at 149.37 cm. This tool works as a roller shade size calculator in metric too — the inside and outside mount math is identical, only the unit changes.
How wide should blinds be compared to the window for an outside mount?
Add twice your chosen side overlap to the window width. A 40-inch window with 2 inches of overlap per side needs a 44-inch blind (44″). There is no deduction on an outside mount — the blind is built at exactly the size you order, so measure the full area you want covered, including the overlap.
What is the blind light gap on an inside mount, and how do I reduce it?
The blind light gap on an inside mount is the operating clearance on each side — Bali's factory spec is 1/8 to 1/4 inch per side. A 36-inch recess with the standard 1/2-inch deduction leaves 0.25 inch per side. You cannot eliminate it on an inside mount without risking the blind rubbing the frame. For near-blackout, switch to an outside mount with generous overlap instead.
Can I reuse old measurements to replace old blinds?
Only if the window frame itself has not changed. When you replace old blinds, re-measure the recess width and height — frames shift, and the old order size already had the factory deduction baked in, so copying it would double-deduct. Re-check the recess depth too, especially if you are switching blind types (a roller shade needs less depth than 2-inch faux wood).
How do I measure for vertical blinds?
This vertical blinds measuring guide follows the same rules: inside mount — narrowest width and shortest height to the nearest 1/8 inch, and the factory takes all deductions (including about 1/4 inch of height on verticals). You need at least 2.5 inches of recess depth to secure them, 3.5 inches for a fully recessed fit. Outside mount — cover the area you want plus 3 inches of overlap per side, so a 60-inch patio opening orders at 66 inches (66″) wide. Use the inside-mount tab above as a venetian blind size guide as well; the math is the same.
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