Blinds Calculator

8 Tips for Measuring Windows for Perfect-Fitting Blinds

Tip 1: Inside Mount vs. Outside Mount Changes Everything About Your Measurements

Inside mount means blinds fit inside the window frame opening, making the frame look larger. Outside mount means blinds extend beyond the frame, usually for better light blocking or to fit an oddly sized window. Inside mount requires precision—measure the width and height of the opening to the nearest 1/8 inch. Outside mount is more forgiving—measure the area where you want blinds to cover. These are completely different measurements, so decide mount type first.

Tip 2: Measure Width at Three Heights—Top, Middle, and Bottom

Window frames sometimes aren’t perfectly rectangular. Measure the width at the top, middle, and bottom of the window. If all three measurements are within 1/8 inch of each other, use the smallest measurement for inside mount (so blinds will fit without binding). If measurements vary more than 1/8 inch, the frame is out of square—you might need outside mount or professional installation. Write down all three measurements immediately.

Tip 3: Measure Height From Top to Bottom—Check for Obstructions

Measure the height of the window opening from the top of the frame to the bottom, in at least two places (left and right side). If measurements differ by more than 1/4 inch, the frame is out of level. For inside mount, use the shortest height measurement to ensure blinds don’t bind. Before ordering, check if the sill depth is deep enough for blinds—some windows have very shallow sills that won’t fit inside-mount hardware.

Tip 4: Account for Depth—Inside Mount Requires Adequate Recess

Inside-mount blinds sit inside the window frame opening. The frame must have enough depth (the distance from the front of the frame to the back) to accommodate the blind mechanism and valance. Standard inside-mount blinds need about 2 inches of recess. Measure the depth of the window frame. If depth is less than 1.75 inches, inside mount won’t work—you’ll need outside mount.

Tip 5: Consider Outside Mount Width Carefully—Blinds Should Cover the Whole Window and More

Outside mount blinds are measured from the wall or trim where they’ll be mounted, not from the frame itself. Measure the width you want the blinds to cover (typically 2-3 inches beyond the window frame on each side for good light blocking). An inside window frame that’s 36 inches wide typically gets 42-48 inches of outside-mount blind width, depending on how much wall space is available and how much light blocking you want.

Tip 6: Measure Depth of the Trim or Wall Area Where Blinds Will Mount

For outside mount, measure how much space extends beyond the window frame—to trim, wall, or architectural features. If a window has deep trim or a wide sill, you have more flexibility in blind placement. If it’s a modern flush-frame window, you might have less than an inch of space. Shallow mounting areas might require recessed outside-mount brackets or very thin blind styles.

Tip 7: Account for Obstacles Like Screens, Grilles, or Hardware

Some windows have divided lights (grilles), which don’t affect blind measurements but affect your visual choice. Some windows have external screens—measure whether blinds go inside or outside the screen. Some have decorative hardware or bars. Identify all obstacles before ordering so you choose the right blind style and mount type.

Tip 8: Order Slightly Undersized Inside Mount, Slightly Oversized Outside Mount

For inside mount: if your measured opening is 36 inches, order 35.75 inches (0.25 inches smaller) to ensure a snug but not too-tight fit. For outside mount: if you want blinds to cover 42 inches, order 42-44 inches to ensure full coverage with no gaps. Blinds expand and contract slightly with humidity and temperature, so undersizing inside mount prevents binding in summer; oversizing outside mount prevents gaps.

 

Where to Go From Here

Now that you understand inside vs. outside mount, frame measuring, width and height variations, recess depth, and obstacle planning, use our Blinds Calculator to determine your exact blind dimensions. Enter your window measurements, choose your mount type (inside or outside), account for any obstacles or special depth considerations, and the calculator will show you the exact blind width and height to order.