Almost every problem with a fitted blind traces back to the measurement, and the measurement is not difficult. It is simply done wrong in a consistent way.
Windows Are Not Square
This is the fact everything else follows from. Frames settle, plaster builds up unevenly, and almost no window in an occupied house is a perfect rectangle.
A single measurement across the middle can be a quarter of an inch wider than the top, which is exactly enough for a shade to jam or to leave a gap.
Measure Three Times, Take the Narrowest
Measure the inside width of the recess at the top, the middle and the bottom. Use the smallest of the three.
This is counter-intuitive because it feels like ordering something too small. It is not: a shade sized to the narrowest point fits everywhere, while one sized to the widest point fits nowhere.
Fit tools that apply the deduction to your narrowest measurement, of the kind at lazblinds.net, exist because this single step is where most orders go wrong.
What Else to Check
- Recess depth, since inside mounts need a minimum, typically around 1.5 inches
- Obstructions: handles, catches, opening mechanisms, trickle vents
- Whether the window opens inward, which changes everything
- Whether the frame is sound, since tension mounts push against it
- Height, measured from the top of the recess to the sill
Understanding the Deduction
Manufacturers subtract a small amount from the stated width so the shade actually fits inside the recess. A shade listed at a given width is made slightly narrower.
This is why you order by your opening measurement rather than subtracting yourself. Doing the deduction twice produces a shade with a visible gap down both sides.
Where a listing states the deduction, read it. Where it does not, ask before ordering.
Use a Steel Tape
A fabric tape stretches and sags, which is enough to matter at this tolerance. A rigid steel tape held against the frame gives a reliable number.
Measure to the nearest eighth of an inch and write all three numbers down rather than remembering them.
Depth Is the Deal-Breaker
A shallow recess is the most common reason an inside mount is impossible. Below the stated minimum depth the shade protrudes, looks wrong and may foul the window.
Checking depth before anything else saves ordering something that cannot be fitted, and it is the point at which curtains or an outside-mount option becomes the honest answer.
Frame Condition
Tension and spring mounts need something firm to push against. Crumbling plaster, flaking paint or a soft repair will let a rod creep over weeks.
Pressing firmly on the frame surface tells you enough. Where it gives or the paint lifts, a tension mount is the wrong choice for that window.
Do It Twice
Measuring every window a second time, on a different day, catches transcription errors and is considerably cheaper than a return.