Choosing the right slat size[^1] for Venetian blinds[^2] feels needlessly complicated. You worry that the wrong choice will ruin your room's aesthetic, fail to provide privacy, and not control light properly.
To choose the right slat size, match it to your window size[^3] and primary need. Use wider slats (50mm+) for large windows to get a better view. Opt for smaller slats (25mm) for standard windows or bathrooms where privacy is the priority.
As someone who works with designers and contractors every day, this is a question I hear a lot. I once worked on a project where the client chose 50mm slats for a small bathroom window. The look was overwhelming and didn't provide the tight closure they needed. Getting the slat size right is about balancing aesthetics, privacy, and light. Let's break down how to make the right choice for your project.
How do you choose the slat size for Venetian blinds?
You're looking at a wall of windows and feel stuck. Should you go with narrow, classic slats or the bold, modern wide ones? The decision can impact the entire feel of the space, and you don't want to make a costly mistake.
For the best result, consider three things: window size, privacy requirements[^4], and your desired view. Large windows suit wide slats[^5] (50mm+). Small windows or areas requiring high privacy, like bedrooms, benefit from smaller slats (25mm-35mm) that close more tightly.
When I advise clients like Emma, an interior designer, we always start with the function of the room. A simple framework can make the choice much clearer. For most situations, we can group the options into three main categories. This approach helps break down the decision into practical terms.
| Slat Size | Best For | Key Characteristics |
|---|---|---|
| Small (e.g., 25mm) | Bedrooms, Bathrooms, Small Windows | Offers the best privacy due to tighter closure. The stack of slats when pulled up is compact. Can look busy on very large windows. |
| Medium (e.g., 35mm) | Living Rooms, Offices, Most Windows | The "classic" choice, especially for wooden blinds. It provides a balanced look, good light control[^6], and is easier to clean than narrow slats[^7]. |
| Large (e.g., 50mm+) | Large Windows, Picture Windows, Patios | Provides the clearest view to the outside when tilted open. Makes a space feel more open and uncluttered. The stack is larger when raised. |
A crucial technical point is the mounting depth. If you plan an inside mount[^8] for a clean look, ensure your window recess is deep enough. For a 25mm slat, you typically need at least 55-60mm of depth to prevent the headrail and blinds from sticking out.
Are wider blind slats better?
You see wide slats featured in design magazines, making rooms look open and airy. This makes you wonder if "wider is always better," but you hesitate, concerned they might not be right for every window in your project.
Wider slats are better for maximizing natural light and offering an unobstructed view, making them ideal for large windows. However, they are not universally superior. Narrower slats provide greater privacy and can be more proportional for smaller windows.
The trend toward wider slats is driven by modern architecture that favors large expanses of glass. On a large picture window, wide 50mm or even 63mm slats look clean and uncluttered. They create fewer horizontal lines, which helps to open up the view. When you tilt them open, you get a much clearer panorama compared to the "busier" look of narrow slats. This is why designers love them for living rooms and spaces designed to connect with the outdoors.
However, there are trade-offs. In a bedroom, the primary concern is often privacy, and smaller slats close more tightly, leaving fewer gaps for light and prying eyes. Wider slats can also overpower a small window, making it look out of proportion. There's a budget consideration[^9] too. From my experience sourcing materials, wider slat profiles often use heavier-gauge aluminum or thicker wood, which can increase the unit cost by 10-20%. Choosing a 35mm slat instead of a 50mm can offer significant savings on a large project without compromising too much on style.
Which blinds block out the most light?
You need to create a dark environment for a bedroom or a media room. You like the style of Venetian blinds but are worried they won't provide the near-blackout conditions you're looking for.
While no Venetian blind can achieve 100% blackout, blinds with smaller slats (25mm) offer tighter closure and block more light than wider-slat versions. For true room darkening, treatments like cellular shades[^10] or roller shades fitted with side channels are superior.
The light-blocking capability of a Venetian blind is all about its closure. Every Venetian blind has small holes for the lift cords, which will always let tiny pinpricks of light through. The main source of light leakage[^11], however, comes from the gaps between the slats themselves.
Here's the key principle I explain to my clients: the narrower the slat, the more slats are needed to cover the window, and the better they overlap when closed. This results in a tighter seal against light. A 25mm slat blind will do a much better job of darkening a room than a 50mm slat blind.
I call this the "light-control spectrum." You can tilt wide slats to flood a room with soft, diffused sunlight. But when you need darkness, you close narrow slats for a near-blackout effect. If a project demands total darkness—for instance, in a high-end home theater—I always recommend moving beyond Venetian blinds. A motorized honeycomb shade[^12] or a dual roller system that pairs a decorative fabric[^13] with a separate blackout layer is the best solution for that specific need.
Do blinds look better inside or outside the recess?
You've finally selected the perfect blinds, but now you face the last decision: how to mount them. Installing them inside the window frame gives a clean look, but an outside mount[^14] seems more practical. You're not sure which is the right call.
An inside mount offers a neat, built-in appearance that showcases your window frame. An outside mount is more forgiving for uneven windows and provides better light blockage. The best choice depends on your window's depth and your design goals.
This is one of the most common installation questions, and there are strong arguments for both. I usually walk my clients through a simple comparison.
Inside Mount:
This is where the blind fits snugly within the window frame. The main benefit is the clean, integrated aesthetic. The windowsill remains usable, which is great for placing plants or decorative objects. However, the big requirement is having enough recess depth. As I mentioned earlier, if the recess isn't deep enough, the blind will protrude awkwardly. An inside mount will also inevitably leave small light gaps along the sides.
Outside Mount:
Here, the blind is mounted on the wall or trim above the window, overlapping the opening. Its primary advantage is light control; because it covers the entire window frame, it eliminates the light gaps you get with an inside mount. It's also a great solution for windows with a shallow recess or an uneven frame. The downside is that it projects into the room, which can sometimes interfere with furniture and looks less streamlined. For my client Emma's projects, we often use inside mounts in living areas for the clean look and outside mounts in bedrooms for superior light blocking.
Conclusion
Choosing the right slat size is a balance of light, privacy, and scale. Use narrow slats for privacy, wide slats for open views on large windows, and the classic 35mm for a versatile, balanced look that fits almost anywhere.
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[^1]: Understanding slat size is crucial for achieving the desired aesthetic and functionality in your space.
[^2]: Explore the advantages of Venetian blinds for light control and privacy in your home.
[^3]: Find out how the size of your windows can dictate the best type of blinds for your space.
[^4]: Learn how privacy needs can influence your choice of window treatments for optimal comfort.
[^5]: Explore the benefits of wide slats for maximizing natural light and views in large spaces.
[^6]: Discover which blinds provide the best light control for different room types and needs.
[^7]: Narrow slats can enhance privacy and light control; learn more about their benefits.
[^8]: Discover the aesthetic advantages of inside mount blinds for a clean, integrated look.
[^9]: Understand how budget considerations can influence your choice of window treatments.
[^10]: Explore the advantages of cellular shades for energy efficiency and light control.
[^11]: Find effective strategies to minimize light leakage and enhance your room's darkness.
[^12]: Discover how motorized honeycomb shades can provide superior light control and convenience.
[^13]: Explore the dual roller system for combining style and functionality in window treatments.
[^14]: Learn how outside mount blinds can improve light control and accommodate uneven windows.
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