Do Wider Shutter Louvres Let In More Light?

It’s a question we’re asked when people are choosing shutters: do wider louvres let in more light? The short answer is yes. The longer answer is more interesting, because the same bit of geometry also changes your view out, how much light gets past them when they’re closed, and even where a mid-rail can go.

Here’s what actually changes as the louvre gets wider, with the measurements behind it.

What louvre size actually means

The louvre size is the width of each slat, measured across its face. Our full height shutters come in five sizes, from 47mm up to 114mm, with 63mm, 76mm and 89mm in between.

The louvres are elliptical in section, thicker in the middle where they pivot and thinner at the edges. The four larger sizes are all about the same thickness at the pivot, around 11mm. That one detail explains almost everything that follows.

Wider louvres sit further apart

A louvre has to be spaced so that, when it closes, it overlaps the one below. So the wider the louvre, the further apart the louvres are set. Measured centre to centre, the spacing is:

Louvre size	Spacing, centre to centre	Louvres in a metre of height
63mm	50.8mm	about 20
76mm	63.5mm	about 16
89mm	76.2mm	about 13
114mm	101.6mm	about 10

Those work out at 2, 2½, 3 and 4 inches.

More light and a clearer view when they’re open

Turn the louvres flat and look straight through them. Each one is only about 11mm thick at the pivot, so what’s left of the spacing is a clear gap.

With 63mm louvres the gap is about 40mm. With 114mm louvres it’s about 91mm, more than twice as tall. Because the louvre thickness stays the same while the spacing grows, a bigger share of the window is open when you look straight through: about 78% of the height with 63mm louvres, rising to about 89% with 114mm.

In practice that means two things. More light reaches the room, and there are fewer bars across your view. With the wider sizes, you’re looking out through a handful of wide openings rather than a fine grille, which is why 89mm and 114mm suit a window with a view worth keeping.

Less light gets through when they’re closed

This is the part people don’t expect. When louvres close, each one overlaps the next by roughly 12mm to 13mm, whatever the size. What light does get through finds its way in at those joins.

A metre of 63mm louvres has about 20 joins. A metre of 114mm louvres has about 10. Fewer joins means fewer places for light to find a way through, so a closed shutter with wider louvres reduces the ambient light level a little further. How much depends on the fit at each join, so treat it as a tendency rather than a measurement.

Louvres move, so there are always small gaps, and any shutter will let some light through. If you’d like less light still, our guide to room darkening covers what you can pair a shutter with.

The other things louvre width changes

The look. Smaller louvres, 47mm and 63mm, give a finer, more traditional look that suits many period properties. The larger sizes look broader and more contemporary.

Depth behind the shutter. A wider louvre reaches further back as it tilts. If your window handles stick out, or the shutter sits close to the glass, there needs to be enough depth for the louvres to turn freely. It’s something we check when we measure, and it can rule a size out on a particular window.

Where a mid-rail can go. A mid-rail has to sit between two louvres, so its position moves in steps set by the louvre spacing. Wider louvres mean bigger steps. If the mid-rail needs to line up with a window transom, a smaller louvre usually gets it closer.

Which louvre size should you choose?

There’s no single right answer, which is why we look at the window before recommending one. As a rough guide:

• For the most light and the clearest view, go wider: 89mm or 114mm.
• For a traditional look in a period property, the smaller sizes often sit better.
• For a bedroom, wider louvres let a little less light through when closed, but pairing the shutter with something else makes more difference.
• For a window with handles close to the glass, the depth may decide it for you.
• For a mid-rail that must line up with the window, a smaller louvre gives more room to get it right.

Not sure which size suits your window?

The easiest first step is to send us a photo of the window with some rough sizes. We’ll come back with an indication of cost. If you’d like to browse first, you can request a brochure.

Frequently asked questions

Do wider shutter louvres let in more light? Yes. Wider louvres are spaced further apart while staying about the same thickness, so the gap between them grows. With the louvres flat, 114mm louvres leave about 91mm of clear gap against about 40mm for 63mm louvres.

What louvre sizes are available for shutters? We offer five louvre sizes: 47mm, 63mm, 76mm, 89mm and 114mm. The size is the width of each louvre across its face, and the right one depends on the window, the room and the look you want.

Are wider louvres better for room darkening? A little. Closed louvres overlap by about 12mm at each join, and wider louvres mean fewer joins, about 10 per metre with 114mm against about 20 with 63mm, so a little less light gets through. Louvres always leave small gaps, so any shutter lets some light in.

Which shutter louvre size is best? It depends on the window and what matters most to you. Wider sizes such as 89mm and 114mm give the most light and the clearest view, while smaller sizes often suit traditional and period homes.

Can any louvre size go on any window? Not always. Wider louvres swing further back as they tilt, so they need more clear depth behind the shutter to turn freely. We check this when we measure, and it can rule out a size on a particular window.