Can You Get Vitamin D Through a Window?

No. Ordinary window glass blocks almost all UVB, the specific wavelength your skin needs to make vitamin D, so sitting by a sunny window does next to nothing for your vitamin D levels. What glass does let through is UVA, the longer wavelength that tans and ages skin. That’s why you can feel warm and even tan a little by a window while making essentially no vitamin D.

It’s one of the most common sun myths, and it matters, because a lot of people assume a bright office or a sunny car seat is topping up their levels when it isn’t.

The short answer, and the science behind it

Sunlight carries two ultraviolet wavelengths that matter for skin: UVB and UVA. Only UVB drives vitamin D production. The catch is that UVB is the one ordinary glass is best at stopping. Standard window glass blocks roughly 90 to 100% of UVB while letting much of the UVA pass through.

So the exact part of sunlight you need for vitamin D is the part the window filters out, and the part it lets through (UVA) does nothing for your levels. This is why “I sit by a sunny window all day” is not a vitamin D strategy, even if the spot feels gloriously warm. To actually make vitamin D, you need direct sun on bare skin with a UV index of at least 3, which is covered in the main guide on how much sun you need.

Why you can still feel hot and even tan through glass

If glass blocks the useful UV, why does a sunny window feel so warm, and why do some people tan next to one? Two reasons.

The heat is mostly infrared, not ultraviolet. Infrared passes through glass easily and is what you feel as warmth, so a sunny window can feel toasty while delivering no vitamin-D-making UVB at all. And the slight tanning or skin changes some people notice come from UVA, which does get through ordinary glass. UVA can deepen pigment and, over time, contribute to aging and damage, all without producing a drop of vitamin D.

So warmth and a faint tan are not signs that your levels are rising. They’re signs of infrared and UVA, neither of which makes vitamin D.

Car windows, office windows, sunrooms: same problem

This applies almost everywhere you sit behind glass, with a small wrinkle for cars.

The takeaway is the same across all of them: you make no meaningful vitamin D behind any standard glass. Windshields happen to block UVA well too (the laminated layer helps), but ordinary side windows and home windows let UVA through, which is the worst of both worlds: no vitamin D, plus the aging wavelength.

The skin-damage twist

Here’s the part people miss. Sitting by a window isn’t just neutral for your skin, it can be a small negative. Because UVA passes through, long hours next to a sunny window, or years of commuting with sun on one side of your face, deliver cumulative UVA exposure. Dermatologists have documented more pronounced aging and sun damage on the window-facing side of regular drivers’ faces. You get the damage side of sun without the vitamin D benefit.

This doesn’t mean you need sunscreen to walk past a window. But if you spend long, regular stretches in direct sun behind glass, it’s worth knowing the exposure is real and the vitamin D payoff is zero.

So when can you actually make it?

To make vitamin D, you need direct sunlight on bare skin, outdoors, with the UV index at 3 or higher, which usually means the middle of the day. No glass in between. A few minutes of real outdoor sun does more for your levels than a whole afternoon in a sunbeam indoors.

If getting outside isn’t practical, glass isn’t the workaround. Food and supplements are. The honest answer for desk-bound days is to step outside at midday when you can, and cover the rest with diet or a supplement.

How Bask helps

Bask is built around the kind of usable sun a window can’t give you. It tracks your live, outdoor UV index and tells you when you’re actually in a window of real vitamin D production, on your bare skin, not the false comfort of a warm seat indoors. So instead of assuming the sunny office is doing the job, you get a clear signal of when stepping outside is worth it and for how long. To see how it compares with other trackers, there’s a 2026 app roundup.

Frequently asked questions

Can you get vitamin D in a sunroom? Not from a standard-glass sunroom. The glass blocks the UVB you need, so a sunroom behaves like any other window: warm and bright, but not a vitamin D source. You’d need to be outside, or have the windows open with direct sun on your skin.

Does UV glass exist? Specialized glazing can be designed to block or transmit specific wavelengths, but ordinary residential and automotive glass is not made to let UVB through. For everyday purposes, assume your windows block the vitamin-D wavelength.

Can you tan through a window? A little, yes, from UVA, which passes through ordinary glass. But that tan comes with no vitamin D and contributes to skin aging, so it’s exposure without the benefit.

Do I still need sunscreen near a window? For brief time, no. For long, regular stretches in direct sun behind glass (a sunny desk, a daily commute), the cumulative UVA is real, and sun protection on exposed areas is reasonable if it’s a daily pattern.

Where to go next

• Start here: How much sun do you need for vitamin D?
• The wavelength that matters: What UV index do you need to make vitamin D?
• The related myth: Does sunscreen block vitamin D?

Sources

• University of Utah Health, “Car Windows Won’t Protect You From UV Rays”: windshield vs side-window UVA transmission.
• NIH Office of Dietary Supplements, Vitamin D Fact Sheet: UVB as the driver of cutaneous vitamin D synthesis.
• Skin Cancer Foundation: glass blocks UVB; UVA exposure and skin damage.