How to Measure for 70 Inch Blinds: Getting the Width Right

A “70-inch” window is a label, not a measurement — measure the actual opening at top, middle, and bottom, and order from the narrowest width you find.

A window sold as 70 inches wide rarely measures 70 inches. Trim, drywall, and framing sit slightly off, and an inside-mount blind cut to the label instead of the opening will rub, bind, or leave a gap. The fix takes about five minutes with a steel tape: three width readings, three height readings, and one decision about inside versus outside mount. Width is the number that ruins orders; a blind slightly too tall can be adjusted, one too wide cannot.

How Do You Measure Width Correctly?

Measure edge to edge at three heights — near the top, middle, and bottom — and record the smallest. Openings are rarely square, and the narrowest point is where an inside-mount blind must fit. Measuring once is the most common cause of a blind that rubs or won’t operate.

Hold the tape tight and level, record to the nearest ⅛ inch, and round down on a 1/16-inch mark, per Blinds.com’s horizontal measuring guide. Always note width first, then height (W × H); reversing them is a quiet, expensive mistake.

Before committing to an inside mount, check depth: measure from the front of the opening to the glass at left, center, and right, and use the smallest figure. Several product lines require a minimum 2½-inch window-well depth, and a handle, crank, alarm sensor, tile edge, or protruding sash can eat into it.

Should You Subtract Anything From the Width?

Usually no — but it depends on the product, and getting this wrong is where most 70-inch orders go sideways. Blinds.com’s general guidance tells inside-mount customers to enter exact opening dimensions with no deductions, because the factory applies the required deduction during production.

Product-specific instructions override that rule. Some blinds ask you to take the narrowest width, subtract ½ inch yourself, and order that number, with no factory deduction. Others list a manufacturer deduction around ⅜ inch, and still others cite roughly ¼ inch with a requirement to round down to the nearest ½ inch.

Read the specifications for the exact blind you’re buying before subtracting a single fraction. Deducting ½ inch on a product that already deducts at the factory leaves oversized light gaps; skipping a deduction on a product that expects one produces a blind too wide to fit.

Once you know your true opening size and which deduction rule applies, our tested 70-inch blind roundup lists which models use which rule.

What Changes for an Outside Mount?

Outside mounts cover the wall area around the opening, so you measure the coverage you want rather than the frame. Add about 3 inches to both width and height where the wall and trim allow — that overlap improves privacy and light control. Check what’s available first: trim, handles, nearby furniture, and the flat mounting surface all limit how far the blind can extend. Blinds.com’s measuring guide covers both mounting styles, including the clearance checks that decide whether an outside mount is even an option.

Where Do Most Measurements Go Wrong?

Three errors account for nearly every bad 70-inch order: measuring once instead of three times, subtracting ½ inch out of habit, and ordering from the nominal label. Two smaller traps: rounding up instead of down pushes an inside-mount blind past the narrowest point, and inside-mounted shades can carry side light gaps by design — some roller and solar-shade instructions cite roughly a 1-inch vertical gap on the cord-loop side and ½ inch opposite, which is normal. Bay windows add one more layer: account for the blind’s actual depth and the clearance between adjacent sections, using the product’s specified fully recessed depth when marking each section.