Measuring windows for blinds is a five-minute job that ruins a three-hundred-dollar order when it's done casually. The good news is that blinds and shades follow the same short rulebook, and every rule exists because of a specific, repeated disaster. Here's the rulebook. (Measuring for curtains is a different discipline with different math; that one lives in our complete curtain measuring guide.)
Decide the mount before touching the tape
Every number you write means something different depending on whether the blind lives inside the window opening or on the wall above it. Inside mount looks built-in, shows off trim, and demands a square opening with enough depth. Outside mount covers the frame, hides crooked openings, blocks more light at the edges, and forgives nearly everything. Decide first, then measure, and label the sheet with the mount type; half the mis-orders we hear about started as an inside-mount measurement ordered as outside, or the reverse.
Inside mount: exact numbers, three points each way
Measure the opening's width at the top, the middle, and the bottom, to the nearest eighth of an inch, and record all three; the maker uses the narrowest. Measure the height at the left, center, and right, and keep the longest. Don't subtract anything: the manufacturer takes their own clearance deduction, and doing it yourself means it happens twice. Then run the depth check: about 2 inches of flat depth for most headrails, closer to 3 if you want the blind fully recessed, with cranks and locks counted as obstructions. Steel tape only; cloth tapes stretch and lie.
Outside mount: coverage, not opening
Outside mount flips the question from "how big is the hole" to "how much do you want covered." The standard answer: 2 to 3 inches past the frame on each side, 2 to 3 above, and to the sill or just below it. More overlap means less edge light, which is why bedroom blinds and anything approaching blackout duty should be generous. Check what's on the wall first, since handles, switch plates, and abutting walls all vote on your coverage before you do.
The five mistakes that generate remakes
- Deducting for clearance yourself, so the deduction happens twice and the blind arrives narrow.
- Measuring one window and multiplying: even matched windows in the same wall vary, and pre-war openings vary by up to an inch.
- Skipping the depth check, then discovering the crank handle the day the blind won't sit flush.
- Rounding to the nearest inch. Makers cut to the eighth; give them the eighth.
- Ignoring an out-of-square opening instead of switching to outside mount, which was always the answer.
When to stop measuring and call someone
Genuinely crooked openings, very tall or hard-to-reach windows, big multi-window orders where one systematic error multiplies, and any project where the blinds are one layer of something larger: these are the cases where professional measuring pays for itself. In Los Angeles and Ventura counties, ours is free, at your home, and it comes with honest advice about whether blinds are even the right product for the window, or whether a roller, zebra, or roman shade would serve the room better.
Frequently asked questions
Do you measure blinds from inside or outside the frame?
It depends on which mount you've chosen, and choosing comes first. Inside mount: measure the opening itself, width at top, middle, and bottom, height at left, center, and right, and report the exact numbers. Outside mount: measure the coverage you want, typically 2 to 3 inches past the frame on each side and above. The single most common ordering error is measuring one way and ordering the other, so write the mount type next to every number.
Do you subtract anything when measuring for inside mount blinds?
No, and this rule prevents the most expensive mistake in the category. Give the manufacturer or workroom the exact opening measurements; they take their own deduction (usually a quarter to a half inch) so the blind clears the frame without rubbing. If you subtract first, the deduction happens twice and the blind arrives narrow, with light gaps you'll see every morning after. Exact numbers, clearly labeled, and let the maker do the math.
How much depth do blinds need for inside mount?
Enough flat, unobstructed depth for the headrail: roughly 2 inches for most faux wood and roller products, closer to 3 for a fully recessed flush look, and more for some cellular and roman headrails. Cranks, handles, and window locks count against you, because the blind has to clear them through its whole travel. Shallow frames aren't a dead end; they're just an outside-mount decision, which older LA windows make constantly.
What if my window opening is not square?
Measure at three points and let the numbers confess. If width or height varies more than about three eighths of an inch, an inside-mount blind will either bind at the tight spot or gap at the wide one, and no amount of careful ordering fixes geometry. Outside mount solves it completely by covering the crooked opening. This is standard procedure in pre-war housing, where openings that measure true are the exception.