How to size window blinds
Measure the surface, then the overhang
Measure the window opening in three places (top, middle, bottom) for width, and in three places (left, middle, right) for height. Use the smallest reading. Inside-mount blinds need 1/4–1/2″ of recess depth; outside-mount add 2–3″ on each side and 3–4″ above for full coverage.
Standard sizes vs. your actual surface
Manufactured "standard" sizes assume textbook measurements. Older homes, custom furniture, and non-standard windows often deviate by 1–3″. If the measurement falls between two stock sizes, choose the larger; it's easier to hem or tuck excess fabric than to add length you don't have.
Mounting and clearance
Inside-mount: deduct 1/4″ from each width measurement so the blinds clear the window casing. Outside-mount: add the overlap on every side. Cordless and motorized mechanisms add about 1″ of stack height — verify the headrail clears the window opening before ordering.
Common mistakes
- Measuring once. Surfaces are rarely perfectly square; take the smallest of three measurements.
- Forgetting shrinkage. Cotton items can lose 3–5% on the first wash.
- Ignoring drape. Heavier fabrics need an extra inch or two to hang correctly.