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About this tool
Turn three width and height measurements into an exact blind order size for inside or outside mount, with depth, squareness and bracket checks.
The Window Blind Size Helper applies the standard made-to-measure blind rule: for an inside (recess) mount you order the NARROWEST of three width measurements and the LONGEST of three heights, and you subtract nothing, because the factory takes its own deduction — typically 12.7 mm for a venetian, 9.5 mm for a roller and 6.4 mm for a cellular blind. For an outside (face) mount it adds an overlap of 38 mm per side, 75 mm above the opening for the brackets and 50 mm below the sill. It also checks whether the recess is square enough and deep enough to take the blind at all.
Open Window Blind Size Helper on AltFTool — it loads instantly in your browser.
Choose Millimetres, Centimetres or Inches, then enter the Width at the top, across the middle and at the sill, and the Height at the left, down the centre and at the right.
Set the Blind type — Roller blind, Aluminium venetian 25 mm slats, Wood or faux-wood venetian 50 mm slats, Zebra / day-night roller, Roman blind, Cellular / honeycomb or Vertical 89 mm louvres — add the Clear recess depth, and tick "Blackout blind — use the wider face-fix overlap" if it applies.
Read the Inside mount order size with the Factory deduction, Clearance each side, Squareness and Depth verdict, take the Outside mount order width and drop if the recess fails either check, then press Copy result.
Narrowest width, longest height — the rule that stops a blind jamming or leaving a gap.
Makes clear the factory removes the clearance, so you do not subtract it a second time and end up with a blind that is too small.
Flags before you order when the reveal cannot take a recess fit.
Measure the width in three places — top, middle and sill — and the height in three places, left, centre and right, using a steel tape. For an inside mount order the narrowest width and the longest height; for an outside mount take the widest width and add at least 38 mm of overlap on each side.
No. Every made-to-measure supplier takes its own deduction off the size you give — usually about 12.7 mm total for venetians, 9.5 mm for rollers and 6.4 mm for cellular blinds — so subtracting a clearance yourself makes the blind roughly a centimetre too narrow. Order the exact recess measurement and let the factory do it.
Roughly 45 mm for a roller or cellular blind, 50 mm for a 25 mm aluminium venetian, 65 mm for a 50 mm wood venetian and 90 mm for verticals if the headrail is to sit fully inside. A shallower reveal can often still take a partial recess fit with the rail projecting, but the minimum for that varies by blind type too: about 25 mm for a roller, cellular or 25 mm venetian, 30 mm for a zebra blind, 32 mm for a 50 mm wood venetian, 40 mm for a roman blind and a full 50 mm for verticals. Below your blind type's own partial-fit minimum, a face fix is the only option.
Inside mount looks neater and leaves the architrave visible, but it needs a recess that is square within about 6 mm and deep enough for the headrail. Outside mount is the safer choice for an out-of-square opening, gives better blackout because the fabric overlaps the frame, and hides an ugly reveal.
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