How to Measure Blackout Blinds | Get Light-Tight Every Time

To measure blackout blinds accurately, choose inside or outside mount, then take three width and three height measurements per window, recording the narrowest width and longest height to the nearest 1/8 inch.

The fix isn’t expensive blinds — it’s precise measurements taken the right way, once. This guide walks through the two mount styles, the exact measuring sequence, the rounding rules manufacturers rely on, and the mistakes that create those annoying light leaks.

Inside Mount or Outside Mount: Which One First?

The mount style controls how you measure. An inside mount fits the blind inside the window recess for a clean, built-in look. An outside mount places the blind on the wall or frame above the window, blocking more light but covering more wall area. Choose inside mount only if the window recess depth is at least 1.5 inches — shallower frames force an outside mount. For sliding glass doors, where the frame depth is often tight, an outside mount is usually the practical route; our tested roundup of blackout shades for sliding doors covers the best options for that setup.

How to Measure for an Inside Mount

Inside mount measurements control the blind’s exact fit within the window frame. Get this right, and the blind sits flush with minimal gaps.

Width (three spots, narrowest wins). Measure the window opening width at the top, middle, and bottom. Record the smallest of the three to the nearest 1/8 inch. This guarantees the blind fits without binding.

Height (three spots, longest wins). Measure from the top of the recess to the sill at the left, center, and right. Record the largest of the three — a shade that’s too short leaves a gap at the bottom.

Depth check. Measure the recess depth at the shallowest point. The required minimum varies by shade type: cellular shades need at least 1 3/8 inches for brackets and 2 1/8 inches for a flush mount; roller shades with a cassette valance need 1 1/2 inches for brackets and 4 3/8 inches for flush mount. If your depth falls short, switch to outside mount.

Squareness check. Measure the diagonals from opposite corners. A difference greater than 1/4 inch means the window is out of square — you’ll need an outside mount or a custom cut to match.

How to Measure for an Outside Mount

Outside mount measurements control the blind’s coverage over the window and wall. The goal is enough overlap to kill every light gap.

Width. Measure the full window width, then add 2.5 inches of overlap on each side (5 inches total). This ensures the blind extends past the frame edges. Round this number down to the nearest 1/8 inch.

Height. Measure from a point 3 inches above the top of the window frame to where you want the blind to end — typically at the sill or 1–2 inches below it. Round this number up to the nearest 1/8 inch.

Wall space. Confirm at least 2.5 inches of flat, clear wall above the window for brackets. Check for obstructions like handles, locks, or window cranks that protrude into the planned blind path.

Light leak prevention.

Common Measurement Mistakes That Cause Gaps

Even careful measurers make these three errors, and each one creates visible light leaks.

• Assuming windows are identical. In the same house, two windows can differ by 1/4 inch or more. Measure every frame individually.
• Making your own clearance deductions. Manufacturers build gap allowances into their cutting process.