How to Measure for Cordless Blackout Shades? | The 15-Minute Method

Getting cordless blackout shades to fit right starts with one decision: inside mount or outside mount. That choice changes every number you record. For an inside mount, measure the window opening and order the narrowest width and tallest height. For an outside mount, measure the coverage area you want spanned, then add overlap on each side to block more light. A steel tape measure and about fifteen minutes are all you need.

Inside Mount: The Three-Measurement Rule

An inside mount sits inside the window frame, giving a built-in look but leaving small light gaps at the edges. For width, measure left-to-right at the top, middle, and bottom; use the smallest of those three numbers. Widths are rarely perfectly square, and the narrowest reading guarantees the shade fits without scraping. For height, measure top-to-bottom at the left, center, and right; use the largest number, since a shade a fraction too short leaves a bottom gap, while one too tall simply rests against the sill. Blindster’s measuring guide for its cordless blackout single cellular shades spells out this pattern—narrowest width, tallest height.

Before ordering, check the bracket depth at the top. A shallow frame that passes the width test can still fail the depth test, so verify this before committing.

Outside Mount: Coverage Plus Overlap

An outside mount brackets the shade onto the trim or wall above the window, covering the entire opening. Measure the width of the area you want covered, then add overlap beyond the window opening on each side for better light blocking.

The outside mount is the better choice when light leakage matters most, as it seals off the side gaps inside mounts always have. Blackout fabric does the heavy lifting, but an outside mount with generous overlap is what actually gets a room close to dark—why many choose it for bedrooms even though the inside mount looks cleaner.

Recording Numbers: Rounding and Deductions

Measure to the nearest 1/8 in and record exact numbers. If rounding, round the width down rather than up—a hair-narrow shade still fits, while a slightly wide one binds in the frame. Blindster’s guides also note not to make your own width deductions for inside mounts; the factory applies its own clearance deductions, so send the exact measurements you took.

Two common errors ruin otherwise good measurements. First, measuring an existing blind instead of the window opening—old shades shrink, warp, and were installed with different tolerances, so they lie. Second, measuring only once—windows aren’t perfectly square, and a single reading misses the narrowest point. Take all three readings for both width and height, every time.

Ordering Checklist Before You Check Out

Run through these four points once your measurements are recorded to avoid the most common return reasons.

• Confirm you recorded the smallest width and the largest height for an inside mount.
• Double-check depth clearance for the cordless mechanism and any window handles or locks that stick into the opening.

When your numbers match those checks, you are ready to order. If you’re still weighing inside versus outside mount and want to see which cordless blackout shades hold up best in real bedrooms, our tested roundup of the best cordless blackout shades breaks down the top options by light blocking and ease of install.