· Michael Blair
Custom blinds are manufactured to the measurements you supply and are generally not returnable. The rules differ between inside and outside mount, and mixing them up is the common error.
Inside mount
Measure the width at top, middle and bottom and use the narrowest. Measure height at left, centre and right and use the longest for most products.
Do not deduct anything — the manufacturer takes the necessary clearance. Deducting yourself produces a blind with a gap down each side.
Outside mount
Measure the area you want covered, then add overlap on each side and above — commonly a couple of inches each way — so light does not leak around the edges.
Confirm there is enough flat surface to mount to above the window, which is where outside mounts most often fail.
- Inside mount: narrowest width, longest height, deduct nothing
- Outside mount: add overlap each side and above
- Steel tape, never fabric
- Measure every window individually, even identical-looking ones
- Check depth for inside mount clearance
Depth matters
Inside mount needs enough reveal depth for the headrail, and it varies by product. A shallow reveal may allow only a partial recess or force an outside mount.
Check the specification before assuming, particularly with cellular shades and shutters which need more depth than a basic blind.
Measure every window
Windows that look identical in a Nashville house rarely are, particularly in older properties. Measure each one and label the measurements by room and position.
That labelling matters at installation, when six similar blinds arrive and need matching to openings.