How Do I Know What Size Roller Blinds to Get?

You're managing a big project, and the window measurements are in. But ordering the wrong size blinds can lead to costly mistakes, project delays, and an unhappy client. It feels like a gamble.

To get the right size, first decide if you want the blind inside the window frame (recess¹) or outside on the wall (exact). Then, use a steel tape measure² to find the width and height. For a recess, use the smallest measurement³ to ensure it fits.

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I see this all the time. The biggest mistake buyers make is assuming the blinds should be the exact same size as the window opening. That's almost never the case. The secret to a perfect, professional finish isn't in guessing; it's in a precise measuring process⁴ and a clear understanding of whether you need a "recess" or an "exact" fit. Let me walk you through how we get it right every single time.

How Do You Accurately Measure a Window for Roller Blinds?

You're standing in front of the window with a tape measure, but you're not sure where to measure from. You're worried one wrong number will ruin the entire order.

For a perfect fit, always use a steel tape measure. Measure the width at the top, middle, and bottom of the window frame, and do the same for the height. For blinds inside the recess, always provide the smallest width measurement to ensure the blind fits.

Getting the measurements right is the most critical step. An old or fabric tape can stretch and give you the wrong numbers, so a metal one is a must. Old window frames are often not perfectly square, which is why we measure in three places.

Inside Mount (Recess Fit)

This is when the blind fits neatly inside the window frame. It gives a clean, minimalist look.

1. Width: Measure the width inside the frame at the top, middle, and bottom.
2. Record the Smallest: Take the narrowest of the three measurements. This is your recess width. If you use the widest measurement, the blind might get stuck.
3. Height: Measure the height on the left, middle, and right.
4. Record the Smallest: Take the shortest of these measurements. This is your recess drop.
   Important: When you order, provide these exact recess measurements. We, the manufacturer, will make the necessary small deductions (usually about 5mm total from the width) to allow the brackets to fit and the blind to operate smoothly. Don't make any deductions yourself.

Outside Mount (Exact Fit)

This is when the blind is installed on the wall or architrave outside the recess.

1. Width: Measure the width of the window opening and add at least 50mm (2 inches) to each side. This overlap prevents light from leaking in around the edges.
2. Height: Decide where the top of the blind will sit (usually 70mm or 3 inches above the frame) and measure down to where you want the fabric to end (usually at least 50mm below the sill). This final measurement is your exact blind size.

Should Blinds Be the Same Size as the Window?

You have your window dimensions⁵. It seems logical to order a blind that matches those numbers exactly, but is that right? You want to avoid awkward gaps or a blind that won't fit at all.

No, blinds should never be the exact same size as the window opening. An outside-mounted blind must be wider and taller to overlap the frame. An inside-mounted blind is made slightly narrower than the recess to allow it to operate without scraping the sides.

This is the single most important concept in sizing blinds. The final look and function of the blind depend entirely on understanding this difference. Whether you choose to mount the blind inside or outside the recess will fundamentally change the measurements you need.

The Logic Behind the Sizing

• Inside Mount: A clean, built-in look is the goal. The blind needs to fit within the architectural frame of the window. However, remember the hardware. The brackets and control mechanism take up space. This means the actual fabric will always be about 30-35mm narrower than the total recess width you provide. This creates small light gaps⁶ on either side, which is a normal characteristic of a standard roller blind.
• Outside Mount: Here, the goal is maximum coverage. By overlapping the window frame, you achieve two things. First, you dramatically improve light control by covering the gaps. Second, you can make a small window appear much larger and more significant. This is also the only option if your window recess is too shallow to hold the blind mechanism⁷ or has obstructions like handles.

So, do roller blinds look better in the recess or in front? It's a design choice. A recess fit is minimalist and sleek. An exact fit⁸ offers better performance for light blocking and can create a bolder visual statement.

Can You Use Ready-Made Blinds for Your Project?

You see ready-made blinds available at a lower cost, and they can be delivered quickly. It's tempting to use them to save time and budget, but will they fit properly?

Ready-made blinds come in standard sizes⁹ that rarely fit a window perfectly. If a blind is too small, it will leave large, unprofessional-looking gaps. For a professional project, a custom, made-to-measure blind is always the superior choice for a guaranteed perfect fit.

On high-end projects, the details matter. A blind that's almost the right size just isn't good enough. While it might seem like a good way to cut costs, the final appearance can cheapen the look of the entire room.

The Problem with "Standard" Sizes

There really is no such thing as a "most common window size." While builders may use similar dimensions, they vary from house to house and country to country. Ready-made blinds are manufactured in rough increments (e.g., 60cm, 90cm, 120cm, 180cm, 210cm). The chances of one of these sizes fitting your specific window recess with the correct clearances are very slim.

What if the blinds are too small?
This is the most common issue. You'll have significant light gaps on the sides, which compromises privacy and light control. The only potential fix is to use an outside mount¹⁰, but only if the ready-made blind is still wide enough to provide the necessary overlap on the frame.

What if the blinds are too wide for a recess?
They simply won't fit between the brackets. While some cheaper blinds can be cut down, I strongly advise against this for any professional installation¹¹. It's nearly impossible to get a factory-perfect, straight cut on the fabric, which can lead to fraying. Cutting the metal tube can also be difficult and can damage the components. The risk and labor involved just aren't worth it when a custom blind costs only slightly more and guarantees a perfect result.

Conclusion

For a perfect, professional finish, always order custom-made blinds¹². Measure the width and drop in three places, and clearly specify if the size is for a recess or an exact fit.

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Smart window treatments shouldn't be complicated. After working with 500+ distributors and contractors worldwide, I've streamlined the process to get you quality products, competitive pricing, and reliable support - every time.

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Paul Chen, Founder
"I built VelaBlinds to solve the real challenges I faced as a project buyer - long lead times, unclear specs, and unreliable suppliers. Let's discuss how we can power your projects with smarter blinds."

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Serving distributors and contractors across North America, Europe, and Australia since 2018.