How to Measure Windows for Blinds | Get the Perfect Fit

For the full breakdown, see our best Cheap Blinds guide.

One wrong measurement and those new blinds won’t fit, letting light leak through or blocking a window crank. Measuring windows for blinds is straightforward when you follow a consistent process. The key is knowing your mount type, using the right tool, and measuring each dimension at multiple points. Whether you’re replacing old blinds or starting fresh, the steps below will get you a perfect fit on the first try.

Inside Mount vs. Outside Mount: Which to Choose?

An inside mount places the blind inside the window frame for a clean, built-in look. An outside mount covers the frame and molding, which hides an imperfect fit and blocks more light. Choose inside mount if your window has at least 1.5 inches of depth and you want the trim visible. Choose outside mount for shallow frames, windows that are not square, or maximum light control.

How to Measure for Inside Mount Blinds

Inside mount blinds sit inside the window frame, so precision at every point is critical. Measure the width at the top, middle, and bottom of the frame. Use the narrowest of the three measurements — the factory will subtract clearance so you do not deduct anything. Measure height at the left, center, and right, then use the longest measurement. If the blind should not rest on the sill, subtract 1/4 inch from the height. For roller shades, record the shortest height instead. Finally, measure the inside depth at the top of the frame to ensure the blind model fits without hitting a crank, lock, or other obstacle.

How to Measure for Outside Mount Blinds

Outside mount blinds cover the entire window opening and molding, so you need full coverage. Measure the outer width of the frame from edge to edge including molding. Add 3 to 4 inches total to this width (1.5 to 2 inches per side) to block light gaps. Measure the height from where the headrail will sit to your desired endpoint — typically the bottom of the frame or a few inches below. Add 3 to 4 inches total to this height. For vertical blinds, add 4 inches to width and 3 inches to height. The extra material is easy to trim later, so round height measurements up.

Common Measuring Mistakes to Avoid

Most fitting failures come from small errors. Use a steel measuring tape only — cloth tapes stretch and slip. Record every measurement to the nearest 1/8 inch. If a measurement falls on a 1/16 inch mark, round width down and height up. Always write the width first, then height (Width × Height) to prevent transposing numbers. Measure every window individually; do not assume two windows in the same room are the same size. Check diagonal measurements from top-left to bottom-right and top-right to bottom-left. If the diagonals differ significantly, the frame is out of square, and an outside mount will give better coverage. For inside mounts, measure only the clear space inside the frame — do not include molding or tracks in your depth reading. If the blind extends to the floor, subtract at least 1/2 inch for clearance so it does not bind against carpet or flooring.

FAQs

What tool should I use to measure window blinds?

Always use a steel measuring tape. Cloth or fabric tapes are not accurate enough for blinds, and they can stretch or slip during measurement, leading to a poor fit. A standard 25-foot steel tape works well for most window sizes.

How much overlap do I need for outside mount blinds?

Add 3 to 4 inches total to the width (1.5 to 2 inches per side) and 3 to 4 inches total to the height. This overlap ensures that light does not sneak in around the edges and that the blind provides full privacy and coverage for the window.

What if my window is not perfectly square?

Measure the diagonals of the window opening. If the two diagonal measurements differ by more than 1/4 inch, the frame is out of square. In that case, an outside mount is usually the better choice because it covers the entire frame and does not rely on a perfectly rectangular opening.