Ordering blinds in the wrong size is one of the most common home mistakes, and it happens because people measure once, in the middle, and order that number. Window recesses are rarely perfectly square, so the width at the top, the middle, and the bottom can differ, and a blind cut to the middle measurement may jam at the narrowest point. This helper fixes that by asking for all three widths and your mount type, then applying the right rule: inside mount uses the narrowest width minus a small deduction, while outside mount uses the widest width plus overlap.
Inside mount means the blind sits within the window recess, and the deduction is what makes it work. This tool deducts 1 centimetre from the narrowest width, which leaves enough clearance for the blind to raise and lower without scraping the sides, while keeping the side gaps small enough to block most light. Some makers apply their own deduction to the size you order, so check the listing: if the maker deducts automatically, order the narrowest width itself, but if they cut exactly what you order, use the deducted figure from this tool.
Outside mount is a different job with a different rule. The blind hangs over the frame on the wall, so it must be wider than the opening to cover it fully, and this tool adds your overlap, typically 5 centimetres, on both sides, plus overlap on the top and bottom of the height. That overlap blocks the light gaps at the edges and hides an uneven frame, which is why outside mount suits windows that are out of square, shallow, or fitted with handles that would block an inside blind.
Here is a worked example with the default inputs. The three widths are 120 centimetres at the top, 119.5 in the middle, and 120.2 at the bottom, so the narrowest is 119.5 and the widest is 120.2, a spread of 0.7 centimetres. For an inside mount, the order width is 119.5 minus 1, which is 118.5 centimetres, and the order height is the measured 150 centimetres. For an outside mount with 5 centimetres of overlap, the order width is 120.2 plus 10, which is 130.2 centimetres, and the order height is 150 plus 10, which is 160 centimetres. The same window gives two very different orders, and the mount type decides which one is right.
Measuring technique matters as much as the maths. Use a metal tape, not a cloth tape that stretches, and measure inside the recess from wall to wall at three heights, holding the tape level. Write down all three numbers before you touch this tool, and do not average them: the average of a recess that is 120 at the top and 118 at the bottom is 119, which still jams at the bottom. If the spread is over 1 centimetre, the recess is noticeably out of square, and an outside mount is usually the safer choice, since it does not depend on the recess shape.
Height needs the same care, and it has its own traps. For an inside mount, measure the height on both sides and use the longer side if they differ, because a blind that is too short leaves a gap at the sill. For an outside mount, decide where the blind will start above the window and where it will end, whether at the sill or below it, and measure that full span. Also check for obstructions: window handles, tiles, and radiators can all force a different mount or a shorter drop, and finding them after ordering is an expensive surprise.
Before you order, run through a final checklist with the tool result in hand. Confirm the mount type matches the listing, confirm whether the maker deducts for inside mount, confirm the order width and height in centimetres, not inches, and confirm the wall or recess can hold the brackets. A blind ordered to the right size, with the right mount, fits first time and moves smoothly for years, while a blind ordered to a single middle measurement often ends up returned, remade, or living with a gap it never should have had.