Article — Blind Size Calculator
The right blind size depends on the mount type. Inside mount uses the smallest of three width measurements and the largest of three heights, all rounded to the nearest 1/8 inch. Outside mount uses the widest measurement plus a 3-inch overlap (4 inches for roman blinds), and again the longest height. Never subtract for light gap yourself — manufacturers deduct 1/4 to 1/2 inch from the inside-mount width during fabrication. Quoting your raw measurements is the safest way to get a blind that fits the first time.
This guide walks through the two mount styles, the three-point measurement method that every major retailer publishes, how each blind type (roller, venetian, roman) tweaks the rule, and the depth and surface checks that decide whether the mount you want is actually possible.
Blind size: inside vs outside mount
Inside mount installs the blind inside the window frame — the headrail sits between the jambs and the fabric hangs in the recess. It is the cleaner, more architectural look, preserves the trim profile, and works well when sill and casing detailing are worth showing. The downside is light leak: there is always a small gap between blind edge and frame, typically 1/4 inch each side after the factory deduction.
Outside mount installs the blind on the wall above and around the frame. The blind is wider and taller than the opening, blocking light leaks on all four sides and visually making the window look larger. It hides shallow or out-of-square frames where inside mount would not fit or would look uneven. The trade-off is a slightly bulkier appearance and a hardware footprint that covers part of the surrounding wall.
The 1/4 inch each-side factory deduction for inside-mount blinds dates to early 20th-century roller-shade manufacturing, when tube assemblies needed enough clearance to spin without scraping the jamb. The figure stuck even as bracket designs evolved — most major U.S. retailers including Lowe's, Home Depot, and Blinds.com still publish 1/4 to 1/2 inch as the standard inside-mount deduction.
The three-point blind size measurement
Houses settle. Window frames warp. Even new construction is rarely square enough that a single measurement across the middle tells the truth about the opening. The three-point method — measuring width at top, middle, and bottom, and height at left, center, and right — catches the variation and lets the calculator pick the safest dimension for the mount you have chosen.
For inside mount the rule is conservative: take the narrowest width so the blind fits the tightest point of the frame, and the longest height so it covers the full opening when lowered. For outside mount the rule is generous: take the widest width and add overlap, take the longest height, so coverage is never short. The asymmetry is deliberate — an inside-mount blind that is too wide jams and bends; an outside-mount blind that is too narrow leaves a permanent light gap.
- Steel tape — cloth and plastic tapes stretch by 1/16 inch over a 4-foot run
- Three widths — top, middle, bottom, measured frame edge to frame edge
- Three heights — left, center, right, measured sill to top jamb
- 1/8 inch precision — rounding finer than this exceeds factory tolerance
- Same person measures all six — reading position changes the result by 1/16 inch
- Write it down immediately — memory drift between rooms costs more than reordering
Why blind size rounds to 1/8 inch
The 1/8 inch (3.2 mm) increment is not arbitrary. It matches the saw kerf and trim tolerance of standard blind manufacturing equipment used by the major North American suppliers. Rounding finer — to 1/16 inch — pushes the order outside their published tolerance, and they will round back up internally before cutting. Rounding coarser — to 1/4 inch — sacrifices precision the workshop is already engineered to deliver.
The calculator rounds inside-mount widths down and outside-mount widths up, with heights always rounded up. That sounds asymmetric but reflects the failure modes: an inside-mount blind that ends up 1/8 inch too wide jams in the frame; rounding down guarantees it clears. An outside-mount blind that ends up 1/8 inch narrower than calculated still has 2 7/8 inches of overlap remaining — well within tolerance.
One of the most common errors is double-deducting: the customer subtracts 1/4 inch each side from their inside-mount width because "you have to allow for clearance", then the factory subtracts another 1/4 inch in production. The blind arrives a full inch narrower than the opening, and light pours in around all four edges. Quote the raw measurements. Manufacturers handle the deduction.
Blind size rules by blind type
Roller blinds are the most forgiving: a continuous fabric tube with no slat geometry to worry about. The standard 3-inch outside overlap (1.5 inches each side) and 1/8 inch tolerance apply directly. Standard widths run from about 12 inches to 120 inches; anything wider needs split or motorized assemblies that change the headrail spec.
Venetian blinds add a complication: the horizontal slats need clearance to tilt without scraping the frame. Standard slats come in 25 mm (1 inch), 35 mm (1.5 inch), and 50 mm (2 inch) widths, with mini-blinds at 13–25 mm. Inside mount is strongly recommended because the close-to-glass installation allows slats to close fully without leaving side gaps. Outside-mount venetians work but tend to look bulky and may show visible gaps at the slat ends when closed.
Roman blinds use a 4-inch overlap (2 inches each side) instead of 3 because the folded fabric stack is wider than a flat roller drop. They also need more frame depth — at least 2.5 inches for the headrail and folded stack to clear the glass at the top of the lift. If the frame is shallower than 2.5 inches, roman blinds go outside mount or get exchanged for roller or venetian.
Roman blinds trace to Ancient Rome, where damp cloths were hung in the Colosseum's vomitoria to keep dust down. Modern roman shades evolved from 18th-century European drapery; the "Roman" name attached in the 1940s American interior-design press. Venetian blinds, despite the name, were invented in Persia and brought to Europe via Venice in the 18th century — the Venetians took the credit because they industrialized the manufacturing.
Depth and clearance checks for blind size
Two non-negotiable checks decide whether your preferred mount type is actually possible. For inside mount, measure the depth from the front of the glass to the inner edge of the window casing. A minimum of 3/4 inch is needed for a roller bracket to clear the glass. For full recess (the headrail fully hidden inside the frame), 2 inches or more is ideal. Below 3/4 inch, switch to outside mount.
For outside mount, measure the flat surface above the window where the headrail will attach. A minimum of 2 inches of flat wall above the casing top is needed for standard L-brackets. Decorative trim, crown moulding, or wall texture below this height will conflict with the bracket footprint. Below 2 inches, either move to inside mount, install an extension mounting board, or relocate brackets to the ceiling.
If your three width measurements differ by more than 1/2 inch, the frame has settled noticeably out of square. The calculator handles the dimension math, but flag this to your installer so they can shim the brackets level. Hanging a perfectly rectangular blind in a parallelogram opening produces visible gaps that look like measurement errors. The fix is bracket adjustment, not blind resizing.
Most U.S. and U.K. retailers accept custom orders from 12 inches to 120 inches per dimension. Outside this range, expect quote-only pricing, longer lead times, and possible refusal — very wide blinds need split or coupled assemblies, very narrow ones may not have a headrail that fits. Confirm availability before ordering if your final dimensions cross either boundary.
Outside-mount venetians block more light overall, but the slats themselves rarely line up flush with the wall surface when closed. The result is a visible 1/4 to 1/2 inch gap at each end of every slat. For venetians, the inside mount produces a tighter, more finished closure even with the small side light gap.