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Blinds Calculator Guide
Blinds are cut to order and non-returnable. Measuring them wrong is expensive and entirely avoidable.
Inside mount: measure width at top, middle and bottom — use the narrowest. Measure height at left, centre and right — use the longest.
Outside mount: add about 3in each side and 3in above the opening.
Your Window Isn't Square
It isn't. Houses settle, frames rack, and a window that measures 35½in at the top can be 35¼in in the middle. That quarter inch is the whole problem.
Order to the widest measurement and the blind won't fit at the narrow point. Order to the narrowest and it fits everywhere, with a barely visible gap where the window is wider. That's why the rule is narrowest for width — you can live with a small gap, you cannot live with a blind that doesn't go in.
Width Narrowest, Height Longest
The two rules point in opposite directions and that's deliberate.
Width uses the narrowest because the blind has to physically pass through the tightest point. Height uses the longest because a blind that's short leaves a gap of daylight at the bottom, while one that's slightly long just rests on the sill. Short is a visible failure; long is nothing.
The Factory Deducts, So Don't
This is where people ruin blinds. For an inside mount you give the manufacturer the exact opening size and they take off the clearance — usually about ¼ to ½in — so the blind runs freely.
If you helpfully deduct half an inch yourself and they deduct another half, you've ordered a blind an inch too narrow, and it's cut to size and non-returnable. Give them the real opening. Let them do their job.
Check the Depth Before You Fall in Love
Inside mounts need enough frame depth for the headrail to sit in. Shallow frames — common on older houses and on many casement windows — simply won't take certain blinds.
Every product lists a minimum depth for a flush mount and a smaller one for a partial. Check it before choosing, because the alternative is discovering it after the box arrives.
Outside Mount Fixes a Lot
If the window is badly out of square, the frame is too shallow, or you want real darkness, mount outside. Overlapping 3in each side hides the frame entirely and kills the light gaps that inside mounts always leak around the edges.
It's the pragmatic choice for bedrooms and for old houses, and it's much more forgiving of a measurement that's slightly off.
Measure in Inches to the Eighth
Round to the nearest ⅛in and use a steel tape, not a fabric one. Write it down at the window, not from memory in the next room — and if you're doing several windows, label each measurement, because "the bedroom one" stops meaning anything by window four.
Related: ceiling fan size calculator, wallpaper calculator, length converter.
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