Can Cellular Shades Be Longer Than Window? A Complete Sizing Guide

You're shopping for cellular shades and the sizing has you second-guessing everything. Should they match your window height exactly, or is it okay if they're a bit longer? Here's the straightforward answer: yes, cellular shades can be longer than your window—and in many cases, they should be.

The length depends on how you plan to mount them and what you want from your window treatment. We'll walk you through the sizing rules, mounting options, and when extra length actually works in your favor.

Why Cellular Shades Are Often Longer Than Windows

Manufacturers design cellular shade fabric to be slightly longer than the ordered drop. This isn't a mistake—it's intentional. The extra material accounts for manufacturing tolerances and helps the shade maintain proper tension over time.

The extra length allows complete coverage when the shade is fully lowered, with the bottom rail resting securely on the windowsill to eliminate light gaps and prevent air convection currents that can compromise thermal performance. That honeycomb structure everyone loves? It only works at peak efficiency when there's a tight seal at the bottom.

If you've ever wondered why your friend's cellular shades seem to have a little extra fabric pooling at the sill, now you know—it's actually doing its job. For our collection of cellular shades, we build in this extra length to make sure you get the insulation and light control you're paying for.

Inside Mount vs. Outside Mount: The Size Difference

Mounting style makes all the difference in how you measure.

Inside mount shades fit within your window frame. Order at the exact opening size with no deductions—the factory will take deductions in production. You measure your window opening precisely, and the manufacturer subtracts what's needed so the shade fits cleanly inside without rubbing against the frame.

Outside mount shades are a different story. Outside mount shades must be ordered significantly longer than the window opening, with an addition of 4 to 6 inches to the window's height measurement to ensure the fabric covers the window trim entirely. You'll also want to add width—typically 2 inches on each side.

Why go bigger? Light control. Outside mount gives you better coverage because the shade overlaps the window frame completely. If you're trying to block light in a bedroom or nursery, this matters more than you might think. Check out our zebra shades and roller shades if you want to compare how different shade styles handle light gaps.

How Much Longer Should Your Cellular Shades Be?

Let's get specific with numbers.

For inside mount: measure your window opening from top to bottom at three points (left, center, right) and use the tallest measurement—this is the number you submit for your window's height. The factory handles the rest.

For outside mount: go at least 2 inches wider than the opening on either side (4 inches total), and for height, go about 3 inches above the window trim and measure down to the window sill. Some folks add even more if they want the shade to extend past the sill for complete blackout coverage.

A 60-inch window? Inside mount stays at 60 inches (you order exact size). Outside mount? You're looking at 66-67 inches to cover trim and eliminate side gaps.

When Extra Length Becomes a Problem

A shade that is drastically too long can occasionally interfere with window operation or look aesthetically awkward if it pools excessively on the floor. We're talking 6+ inches of fabric bunching up at the bottom when fully lowered.

If your cellular shade is only slightly too long, it's usually fine. The cellular material is designed to compress without damage, so simply letting the fabric rest on the sill is often the easiest remedy. But if you've got serious excess causing the fabric to drag or buckle, you have a real issue on your hands.

Professional alteration is the safest fix. Due to the delicate nature of the cellular fabric and internal cord routing within the cells, attempting to cut or shorten a cellular shade at home is highly discouraged. One wrong snip and you've damaged the insulation properties that make cellular shades worth buying in the first place.

Measuring Tips to Get It Right the First Time

Use a steel tape measure, not a cloth one. Measure down to the nearest 1/8 inch. Round up if you're between marks—better slightly larger than slightly short.

Measure each window individually—never assume that your windows are the same size, even if they may look alike. Old houses settle. New construction isn't always perfectly square. Those "identical" windows in your living room? Probably off by at least a quarter inch.

For inside mount, you need to check window depth too. Determine if window casing has enough depth for an inside mount window treatment by measuring from the front of the opening to the glass. Most cellular shades need at least 2 inches of depth, sometimes 3 for a flush fit.

Comparing window treatment options? Our roman shades and shangri-la sheer shades have different depth requirements, so it's worth exploring your options if space is tight.

Making Cellular Shades Work for Your Space

When you close your window treatment it will cover the entire window, so it blocks more light than window treatments mounted on the inside of the window casing. This is exactly why people choose outside mount for bedrooms and home theaters—the extra length and width eliminate those annoying light halos around the edges.

But inside mount has its place too. Inside mount is perfect if you want to highlight the decorative trim around your windows or you have a deep sill that you can use as a shelf. You lose a tiny bit of light blocking compared to outside mount, but you gain that clean, built-in look.

Still comparing your options? Read our guide on cellular shades vs solar shades to see how different shade types handle light, heat, and privacy differently.

Conclusion

Cellular shades can absolutely be longer than your window, and in most outside mount situations, they should be. The key is understanding which mounting style fits your needs and measuring accordingly. Inside mount shades are ordered at exact window size (the factory makes them slightly smaller), while outside mount shades need 4-6 inches of extra length and width to cover trim and block light effectively.

Measure carefully, use the right mounting style for your space, and don't stress if your shades have a bit of extra fabric at the bottom—that's often exactly what you want for proper insulation and light control.

FAQs

How much smaller do manufacturers make inside mount cellular shades?

Manufacturers typically deduct 3/8 inch from the width you order for inside mount cellular shades. This gives clearance so the shade operates smoothly without scraping the window frame. You provide the exact window opening measurement, and they handle the deductions automatically during production.

Can I install cellular shades that are 2 inches longer than my window?

Yes, if you're doing an outside mount. The extra 2 inches helps cover window trim and reduce light gaps. For inside mount, though, 2 inches too long would cause the shade to bunch awkwardly at the sill and potentially interfere with proper operation.

What happens if my cellular shades are too short?

Shades that are too short leave gaps at the bottom, allowing light and air to leak through. This defeats the insulation benefits of the honeycomb design. If they're only slightly short, you can lower the mounting brackets, but this leaves a gap at the top. Replacement is usually the better option.

Do I need different measurements for blackout cellular shades vs. light filtering?

No, the measuring process is the same regardless of opacity. However, if you want complete blackout, outside mount works better because it covers more of the window frame and eliminates side light gaps that are more visible with blackout fabrics.

Can cellular shades be adjusted if they're too long?

Minor excess length (1 inch or less) usually isn't a problem—the fabric can rest on the sill without damage. For anything more than that, professional shortening is recommended. DIY cutting can damage the honeycomb cells and internal cords, voiding warranties and ruining the shade's insulating properties.