Window Blinds Calculator

Window blind sizing requires precise measurements of the window opening and a clear decision between inside mount (within the frame) and outside mount (over or around the frame).

The one question to answer before you order anything: who makes the deduction?

An inside-mount blind has to be slightly smaller than the opening, commonly a quarter to a half inch off the width in total (not per side) and about half an inch off the drop, so it raises and lowers without rubbing the jambs. That part is not in dispute. What varies is who subtracts it, and there are two live conventions:

• The supplier deducts. You measure and order the exact opening. This is what Blinds.com, Select Blinds and most large US retailers instruct, usually in bold on the order form, and it is the default here.
• You deduct. You order the finished blind size and the factory makes exactly what you asked for. Some smaller makers and most trade suppliers work this way.

Get it backwards in the first direction and the deduction happens twice: your 36 inch window gets a 35½ inch blind with a quarter-inch gap down each side. Get it backwards in the other and it will not go into the recess at all. Custom blinds are not returnable, so this is the field to read the order form for.

Inside mount, both ways

Order the opening: Blind Width = Window Opening Width Order the finished size: Blind Width = Window Opening Width − 0.25 in Order the finished size: Blind Length = Window Opening Height − 0.5 in

The calculator prints whichever the supplier wants and shows you the other figure alongside it, so the two can never be confused.

Measure at more than one point. Old houses are rarely square. Take the width at top, middle and bottom and use the narrowest; take the height at left and right and use the tallest. On an outside mount the opposite logic applies, since you are covering the frame rather than fitting inside it.

Outside Mount Formulas:

Width = Opening Width + (2 × Side Overlap) Coverage only: 1.5 to 2 inches per side. Blackout: 3 inches or more per side.

Length = Opening Height + Top Overlap + Bottom Overlap Coverage only: 1 to 2 inches above the frame. Blackout: 3 inches or more.

This calculator uses the blackout figures, 3 inches per side and 3 above, because outside mount is usually chosen for light control in the first place.

Does the frame have room for an inside mount? This is the check people skip. An inside mount needs enough flat depth in the jamb to house the headrail, and the headrail is deeper than you think.

Roller and cellular blinds: about 2 inches minimum. 1-inch aluminium venetians: about 1.5 inches. 2-inch faux wood venetians: 2.5 to 3 inches, more if you want a full recess with nothing protruding.

Measure from the face of the window glass to the front edge of the jamb. If you have less than the headrail needs, you have three options: a partial inside mount where the rail sticks out slightly, a shallower blind type, or an outside mount. Ordering an inside mount for a jamb that cannot take it is the single most common custom-blind mistake, and custom blinds are not returnable.

Worked example, inside mount: Window opening: 36.25 inches wide × 48.5 inches tall, using a 0.25 inch deduct. Blind width: 36.25 − 0.25 = 36 inches Blind length: 48.5 − 0.5 = 48 inches

Worked example, outside mount for light control: Same window opening: 36.25 × 48.5 Add 3 inches per side: 36.25 + 6 = 42.25 inches Add 3 above and 1 below: 48.5 + 4 = 52.5 inches

Measuring tips:

• Measure width in 3 places (top, middle, bottom) and use the smallest measurement for inside mounts
• Measure height in 2 places (left and right) and use the longest for inside mounts
• Always measure to the nearest 1/8 inch: blind manufacturers work in 1/8-inch increments