How to Measure for Roller Shades | Do It Right the First Time

Measure roller shades by width first, then height, to the nearest 1/8 inch, using the narrowest width and tallest height for inside mounts.

Getting measurements wrong is the most common cause of frustrating returns. Whether you’re mounting inside the window frame for a clean look or outside for better light blocking, the process is straightforward if you follow a consistent order. The golden rule: width first, height second, and always measure to the nearest 1/8 inch — never round up to the nearest half inch.

Before picking up a tape measure, decide your mount type. This choice changes where you measure and how much overlap you need. Inside mounts sit within the window frame; outside mounts cover the frame and wall around it.

Inside Mount: The Three-Point Rule

For an inside mount, the shade fits snugly within the window opening, so precision matters more. Measure the width at the top, middle, and bottom, and the height at the left, center, and right. Use the narrowest width and the tallest height from those readings — this ensures the shade fits without binding while covering the full vertical span.

Before ordering, check the window’s depth. Many roller shade brackets need a minimum mounting depth, often around 1.5 to 2 inches, to sit properly. A shallow frame can leave hardware sticking out or prevent installation. Common inside-mount mistakes include ignoring protruding sills, handles, or knobs, and assuming the frame is perfectly level — it rarely is, which is why the three-point method exists.

Outside Mount: Cover the Whole Area

An outside mount covers the entire window opening plus some wall space. Measure the full width and height of the area you want covered, then add overlap on each side. Most guides recommend roughly 1.5 inches per side for standard coverage, and 2–3 inches per side when light blocking is the priority. For a 36-inch-wide window, that means a shade width of 39 to 42 inches depending on your overlap choice.

For height, measure from the top mounting point down to where you want the shade to end. If mounting above the trim, measure from the header or mounting surface, not the window opening itself. The key is consistency: use the same reference line for everything.

What Most People Get Wrong

Even careful measurers make predictable errors. The most common is mixing up width and height — width is always horizontal, height vertical. Recording in the right order matters because orders use the width-by-height format: a 36-inch-wide by 48-inch-tall window is written as 36″ x 48″. Failing to measure each window separately is another frequent mistake; identical-looking windows can differ by a quarter inch or more due to settling or framing variations.

Rounding errors are equally costly. Always record to the nearest 1/8 inch, even if your tape shows a fraction like 35 7/16 inches. Before finalizing any inside mount, re-check for obstructions — crank-out windows, window guards, or thick paint layers inside the frame can reduce usable space. If you’re ready to compare specific sizes, our roundup of the best 72 inch wide roller shades covers tested options for larger windows.

Quick Reference: Inside vs. Outside Mount

The guide also advises checking for protruding sills or trim that could interfere with an outside mount’s overlap.

Before You Order: Final Checklist

You’ve measured and re-measured. Run through these final checks before submitting. Confirm the mount type matches your hardware — inside mounts need depth clearance, outside mounts need enough flat wall space for brackets. Verify every window gets its own measurement, even if they look identical. Double-check that width and height are in the right order, and that fractions are to the nearest 1/8 inch. If your window has unusual features like deep sills or uneven framing, note them in the order comments so the manufacturer can flag fit issues before cutting your shade.