How to Measure Windows for Blinds

Almost every ill-fitting blind traces back to the same moment: a rushed measurement. Because made-to-measure blinds are cut to your figures and usually can’t be returned, five careful minutes with a tape measure is the most valuable part of the whole project. Here is how to do it properly.

Use the right tool

Reach for a steel tape measure, not a cloth or plastic one. Cloth tapes stretch and sag, and even a couple of millimetres of error can leave a blind that binds in the recess or lets light spill down one side. A rigid steel blade sits flat against the frame and gives you a repeatable reading. Record every measurement in the same unit — pick either inches or centimetres and stick with it — and write to the nearest eighth of an inch or single millimetre.

Decide the mount first

You cannot measure sensibly until you know whether the blind will sit inside the window recess or outside it on the wall or frame. The two are measured completely differently: an inside mount is measured to the exact opening so the factory can trim a small clearance, while an outside mount is measured to the coverage you want, with extra added on every side. If you are undecided, read our guide to inside versus outside mounts before you pick up the tape.

Measuring for an inside mount

Windows are rarely perfectly square, so you take several readings and let the smallest win:

• Width: measure across the recess at the top, the middle, and the bottom. Use the narrowest of the three — a blind sized to the widest point will jam.
• Height (drop): measure from the top of the recess to the sill on the left, the centre, and the right. For an inside mount most people use the longest so the blind reaches the sill.
• Depth: measure how deep the recess is. Every blind has a minimum depth for a flush inside fit; if yours is too shallow, the headrail will protrude or you’ll need an outside mount.

Enter your exact opening figures into the Blind & Shade Size Calculator and it applies the standard clearance deduction for you, so you can see the size to order at a glance.

Measuring for an outside mount

Here you are deciding how much of the wall the blind should cover. Measure the opening, then add overlap: a common rule is about 4–5 cm (1.5–2 inches) on each side and a similar amount above, which hides light gaps and makes the window look larger. Make sure there is a flat, solid surface to screw the brackets into above the opening, and enough room for the stacked blind to clear the glass when raised.

Common mistakes to avoid

A handful of errors account for most sizing disasters:

• Measuring only once. Always take the width in three places and the height in three; an out-of-square window is the norm, not the exception.
• Mixing units. A width in inches and a drop in centimetres on the same order form is a recipe for a costly mistake.
• Deducting twice. Some retailers want the exact opening and deduct the clearance themselves; others expect the finished size. Check which, and never deduct on top of a factory that already does.
• Ignoring handles and obstructions. Tilt-and-turn handles, cranks, and deep tile reveals all affect whether an inside mount will clear.

Write it down and double-check

Note each window’s width, drop, depth, and chosen mount on paper or your phone, label it by room, and measure a second time before you order. Treating the numbers as final only after that second pass is the single habit that separates blinds that drop in perfectly from the ones that go back in the box. Once your measurements are locked in, the size calculator turns them into an order size in seconds.