How to Measure Window Blinds? | Inside & Outside Mount Steps

Getting window blind measurements wrong means reordering or dealing with gaps that let light pour in. The right way to measure window blinds takes about ten minutes with a metal tape measure and a notepad, whether you’re fitting blinds inside the frame or covering the whole window from outside. The process differs slightly for each mount style, and the details matter more than most people expect.

What You Need Before Measuring

Grab a metal or steel tape measure. Cloth tapes stretch and give you distorted numbers that lead to blinds that don’t fit. You’ll also want a pencil and paper to record each reading — don’t trust memory for this.

While you’re at it, inspect the window recess for anything that could block a blind: locks, levers, handles, cranks, molding, or window tracks. Levolor’s measurement guide specifically calls out “Window Obstacles” as a factor you must account for before ordering. Depth measurement must exclude any hardware sticking out from the frame.

Inside Mount: The Exact 1/8-Inch Method

For blinds that sit inside the window frame, measure width and height in three spots each, then record the values that guarantee a fit.

Measure the inside width horizontally at the top, the middle, and the bottom of the frame. Measure the inside height vertically at the left side, the center, and the right side.

Finally, measure the depth from the front edge of the frame to the window glass. This tells you whether the blind style you want can physically fit in the recess. A shallow frame rules out certain products, so check this before you order, not after.

Outside Mount: Cover the Whole Area

Outside-mount blinds attach to the trim or wall above the window, so you’re not measuring the opening at all. Measure the overall area you want the blind to cover, not the interior frame. For width, measure horizontally between the outermost points where the treatment will sit — typically the outside edges of the trim or casing.

The table below summarizes the key differences between the two methods.

Mistakes That Ruin a Blind Order

Measuring only once is the most common error. Window frames are rarely perfectly square, and a single reading can miss a bow or tilt in the frame. The three-point method exists precisely because frames vary, and both Levolor’s window blind measuring guide and Home Depot’s instructions require it.

Using the wrong width value is the second big miss — for inside mounts, it’s the narrowest width and the longest height, not the average and not the widest. Rounding up to a friendlier number instead of keeping the 1/8-inch precision is the third. And skipping the depth check entirely is the fourth, which is how people end up with a blind that physically can’t sit in the recess.

If you’re ordering for a bathroom, where moisture and privacy change the equation, see our tested picks for the best bathroom window blinds before you commit to a style.