Blind Size Calculator

Ordering a blind is a two-part problem: the finished panel size and the depth of the pocket it has to live in. An inside mount sits within the window reveal, so the manufactured width is cut slightly under the tightest opening measurement, conventionally by a quarter to three eighths of an inch, to leave running clearance for the brackets and end caps. An outside mount is fixed to the casing or wall and is deliberately oversized, normally by about 1.5 inches on each side and above, so that light does not leak around the edges. Because window openings are rarely square, the reveal is measured at three points: the narrowest of the three widths governs an inside mount, and the longest of the three heights prevents a gap at the sill. Depth is what decides whether an inside mount is possible at all. A roller tube needs roughly 1.5 inches, cellular and pleated headrails about 1.75 to 2 inches, aluminum and faux wood Venetian headrails 2 to 2.5 inches, and vertical headrails 3 inches or more. The main caveats are obstructions inside the reveal such as crank handles, alarm contacts and tilt-and-turn hardware, and the fact that an inside mount always leaves a light gap at the slat edges, so a blackout requirement usually points to an outside mount. Fit also affects performance: the Department of Energy attributes roughly 25 to 30 percent of residential heating and cooling load to windows, and a tightly fitted cellular shade traps a still air layer that a loose one does not.

The slatted blind reached Europe from Persia by way of Venice, which is how the English name Venetian blind arose. A London patent for such a blind was granted to Edward Beran in 1769, and the type spread quickly through Georgian townhouses and, later, public buildings in the United States, where John Hampson of New Orleans patented an improved mechanism for tilting the slats in 1841. Roller shades developed on a parallel track from the plain Holland linen blind of the same era, a stiffened cloth wound on a wooden roller. For most of the nineteenth century blinds were carpentry, made to the opening on site. Mass production changed the sizing conventions. Aluminum slat blinds introduced in the 1940s made narrow slats cheap and turned the blind into a catalogue product ordered by dimension rather than built in place, which is when the fractional deduction for inside mounts became a printed rule on order forms. The honeycomb or cellular shade, marketed from the mid-1980s, added an insulating air cell and its own headrail depth requirement. The most recent change is regulatory rather than dimensional: after decades of reports of child strangulation on pull cords, the industry standard ANSI/WCMA A100.1 was revised in 2018 to require that stock window coverings sold to consumers be cordless or have inaccessible cords, which is why current order forms ask for lift type as well as width, height and mount.