🪟 Blind Size Calculator

How to Use the Blind Size Calculator

Measure your window opening in inches, choose inside or outside mount, and press Calculate. For inside mount you get the order size with the factory clearance deduction applied. For outside mount you get the size with overlap added. Write the numbers down as width × height, since that is how blinds are ordered.

Measure with a steel tape, not a cloth one, and record to the nearest eighth inch. For inside mounts, measure the opening at three widths and three heights; enter the narrowest width and the longest height. Round nothing: the factory handles deductions, and your job is exact raw numbers.

The Measuring Rules

Inside-mount blinds are made slightly narrower than the opening so they operate without scraping; the half-inch deduction is standard across the industry. Height stays exact because the blind hangs to the sill. Outside mounts add overlap to kill light gaps and hide imperfect trim, which is why bedrooms and media rooms favor them.

Worked Example

A 36 × 48 inch window opening. Inside mount: order 35.5 × 48 inches; the factory builds in the clearance. Outside mount: order 40 × 52 inches, giving 2 inches of overlap per side and top/bottom. The outside blind covers the trim completely and blocks the edge light that inside mounts always leak slightly.

Inside vs. Outside Mount: Choosing

Inside mount looks tailored and shows off trim work, but it demands a square opening and enough frame depth (2+ inches for most blinds). It also leaks a thin halo of light around the edges, since the blind must clear the frame. For bedrooms, that halo matters; blackout needs outside mount or side channels.

Outside mount forgives crooked old windows, covers dated trim, and maximizes darkness and privacy. The tradeoff is a less built-in look and a blind that protrudes into the room. For shallow frames, rental-friendly no-drill options, or windows that are visibly out of square, outside mount is the practical answer.

Common Measuring Mistakes

The top mistake is measuring once in the middle. Openings taper; a blind ordered to the middle width jams at the narrower top. Always take three measurements and use the extremes. Second: confusing width and height when ordering, especially for horizontal versus vertical blinds. Say "width by height" out loud as you write it.

Third: measuring the old blind instead of the window. Old blinds may themselves be wrong, and replacements copy the error. Fourth: forgetting obstructions like cranks, handles, and alarm sensors that the blind must clear. Note the projection depth of anything protruding into the opening before choosing inside mount.

Special Windows

Bay windows need each section measured separately, with attention to how the blinds meet at angles. French doors take door-specific blinds that mount to the door and clear the handle; measure the glass plus desired overlap. Arched tops need specialty shapes; measure the rectangle portion and the arch rise separately.

For very wide windows over 72 inches, consider two blinds on one headrail for smoother operation and easier handling. Sliding doors suit vertical blinds or panel tracks measured to the full door width plus stack space. When a window truly defies standards, custom is cheaper than the wrong stock size twice.

Ordering and Installation Tips

Double-check every number before submitting; custom blinds are final sale. Order all blinds for one room together so the dye lots match. When they arrive, dry-fit before drilling: hold the blind up and confirm width and operation. Most mounting brackets need only a drill, level, and pencil.

Mount into studs or use proper wall anchors; a wide wood blind is heavy and drywall alone will not hold it. Keep cords out of reach of children and pets, or choose cordless: cordless costs more but the safety and clean look are worth it. Adjust the slat angle seasonally to bounce daylight deep into the room.

Blind Types and Light Control

Horizontal blinds (wood, faux wood, aluminum) tilt for precise light control and raise fully for a clear view. Cellular shades insulate best, trapping air in honeycomb pockets, and stack tight. Roller shades give the cleanest look and the best blackout options. Vertical blinds suit sliding doors but sway in drafts.

For bedrooms, pair any blind with blackout curtains or choose blackout-rated shades; slatted blinds always leak some light. For street-facing rooms, top-down/bottom-up shades give privacy while keeping daylight. Match the product to the room job, not just the window size.

Worked Example: The Old Crooked Window

A 1920s double-hung, opening 34 x 60 inches, out of square by half an inch, frame depth only 1.5 inches. Inside mount is a bad bet: the crooked opening would show uneven gaps, and 1.5 inches of depth leaves most blind headrails sticking out. Go outside mount. Width: 34 + 4 = 38 inches, two inches of overlap per side to kill light gaps. Height: 60 + 4 = 64 inches, two inches above for the mounting brackets and two below to cover the sill.

If the frame were a true 3 inches deep and the opening square, inside mount would win: measure width at top, middle, and bottom, order the narrowest minus the factory's standard clearance (usually half an inch total), and keep the height exact. The decision is never about looks first. It is about what the window allows, and old windows allow less than you hope.

Measuring Mistakes That Cost Real Money

Custom blinds are final sale at most retailers, so measuring errors become wall art. The expensive ones: measuring the trim instead of the opening, which adds inches that do not exist; taking one width measurement on a window that is wider at the bottom than the top, which old houses do routinely; and ordering an inside mount at the exact opening size, forgetting the clearance the blind needs to operate, so it wedges and will not tilt.

Use a steel tape, not a cloth one, and measure every window separately even when they look identical; builders in the same house routinely vary by a quarter inch. Write width first, then height, every time, because the industry standard is width x height and a transposed order ships a blind that fits nothing. And for inside mounts, check frame depth before falling in love with a style: most horizontal blinds need 2 inches of depth, and cellular shades can work with less. Depth decides the shortlist, not taste.

Doors and French Doors: The Special Case

Glass doors break the standard rules because the blind mounts to the door itself and has to clear the handle. Measure the glass plus the frame you want covered, then add 2 inches of overlap on each side so no light leaks past the edges. The critical number is handle clearance: hold the blind's bottom rail depth against the door and make sure the handle does not hit it. Most door blinds need at least 1.5 inches of clearance, and chunky lever handles often need a 2-inch spacer or an outside-mount frame around the glass.

Tie-downs are the detail people skip. A blind on a door swings and bangs every time the door moves unless the bottom rail is anchored with hold-down brackets. They cost a few dollars and take five minutes to install, and they are the difference between a blind that looks built-in and one that looks like an afterthought. Measure the door, mind the handle, anchor the rail.