How this calculator works
A window recess is not square, and a roller or Venetian blind that is ordered to the average measurement will either jam or leave a gap. Measure the width at the top, middle and bottom and the height on the left, centre and right. For an inside mount the blind has to clear the narrowest point, so the order width is the smallest width minus a small clearance, and the drop is the shortest height. For an outside mount the blind must cover the whole opening plus a margin, so the order size uses the largest measurements and adds an overlap.
Formula
inside mount: width = minimum of the three widths - side clearance. drop = minimum of the three heights. outside mount: width = maximum of the three widths + 2 x overlap. drop = maximum of the three heights + overlap.
Worked example
Widths of 120.0, 120.4 and 120.2 cm and heights of 150.0, 150.3 and 150.1 cm give an inside width of 120.0 - 0.5 = 119.5 cm and a drop of 150.0 cm. The same window as an outside mount with a 5 cm overlap needs a blind 130.4 cm wide and 155.3 cm long.
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Frequently asked questions
Why subtract a clearance instead of using the exact narrowest width?
A blind has brackets and a fabric roll or slats that need room to move. The manufacturer usually states the deduction in its measuring guide, so use that figure if it differs from the default.
Which mount should I choose?
An inside mount looks cleaner and leaves the window frame visible, but it needs a recess deep enough for the brackets. An outside mount covers more light and suits a shallow recess or a window without a frame to fix into.
Does the drop include the headrail?
Most suppliers quote the drop as the fabric length excluding the headrail, or state which. Check the product guide, and add the headrail if you want the blind to reach the floor exactly.
What if the shortest height is the centre rather than a side?
The calculator uses the smallest of the three regardless of where it is measured. That is the safe approach for an inside mount, because a blind longer than the recess will not hang.