Does sunlight through a window give you vitamin D?
Sunlight through a window does not give you vitamin D because glass blocks the UV-B wavelengths your skin uses to make it. The warmth you feel is infrared, and the brightness is visible light. Neither makes vitamin D. A sunny home window, car windshield, or office window can feel bright and warm while producing no vitamin D in your skin.
Why does glass block vitamin D?
Glass blocks vitamin D because the one band your skin needs, ultraviolet B at roughly 290 to 320 nanometres, does not pass through it. Everything else about sunlight comes through fine. The visible light that brightens the room and the infrared that warms your face both come through. The narrow slice of the spectrum that actually starts the reaction in skin does not.
That mismatch is what makes the experience so misleading. Warmth and brightness are the two cues everybody uses to judge how much sun they are getting, and neither one carries the wavelength that matters. The patch of sun on the floor is real sunlight in every ordinary sense. It is simply missing the part of sunlight that vitamin D is made from.
What about car windows, and does it depend on the glass?
A car windshield, an office window, and ordinary home window glass all give the same answer for vitamin D, and the answer is none. Nothing about the particular pane changes it. A windshield that turns the cabin into an oven is passing infrared, and a spotless window that floods a room is passing visible light. Glass blocks the UV-B your skin would need.
The same fact explains why the small adjustments people make do not help. Moving your chair closer to the pane, taking the sunniest seat, or opening the blind wider all increase the light and the heat you can feel, and none of them restore the wavelength that is missing. For vitamin D, time behind glass counts as zero, however strong the sun looks or feels from that seat.
So what does actually make vitamin D?
Vitamin D is made in skin by UV-B, and by nothing else in sunlight. Whether that band is reaching the ground at all comes down to how high the sun is rather than how bright the day looks, which is why the answer changes with season and latitude even under a clear sky.
What the mechanism never delivers is a personal number. People differ enormously in how much vitamin D the same ultraviolet dose produces in their blood, so nobody can read your result off a window, a forecast, or an hour spent outside.
One thing is settled enough to act on without any of that. Correct sunscreen use does not prevent vitamin D synthesis and does prevent burning, so there is never a reason to skip sunscreen for vitamin D.
So should you go outside for vitamin D?
Going outside is a reasonable thing to do, and the window fact is not what decides it. What the glass settles is where the reaction cannot happen. It says nothing about how much you need, or whether you already have enough.
What it can do is tell you when the day is gentler, so read the UV Index rather than the brightness: it tells you when protection starts at 3, when the day peaks, and when it falls back.
If vitamin D itself is the worry, that is a question for a clinician, not a calculation you can run on a window.
Where do I start?
- Stop counting the sunny desk, the window seat, and the long drive as vitamin D. For that purpose they are all zero.
- Notice which cue you were actually using. Warmth and brightness feel like evidence and are not.
- Read the UV Index as a clock when you do head out: when it crosses 3, when it peaks, and when it drops back. Cover up inside that window and take your uncovered time outside it.
- Keep your ordinary sun-protection habits outdoors, and never skip sunscreen for vitamin D. Correct use does not prevent synthesis.
- Keep two questions apart: whether your skin can make vitamin D here, and whether this is a bright place to sit. Glass settles the first one only.
- Take a real vitamin D concern to a clinician rather than trying to estimate it from how much sun you think you got.
What does the science say?
Common questions
Can you get vitamin D through a window?
No. You cannot get vitamin D through a window because glass blocks the UV-B wavelengths that trigger vitamin D synthesis in skin. Bright visible light and infrared warmth can still pass through and feel strong, but neither makes vitamin D.
Does an open window make a difference?
An open window removes the pane, and the pane was the whole objection, so the glass fact stops applying to sun that comes through the opening. Everything else still holds. Whether UV-B reaches you depends on how high the sun sits rather than how bright the gap looks, and the same ultraviolet produces very different results in different people, so nothing here converts into a personal amount.
Can you get vitamin D while sitting in a car?
No. A car windshield stops the UV-B, so a drive in full sun produces no vitamin D no matter how long it lasts. What a hot, bright cabin proves is that infrared and visible light are coming through, and those are the two parts of sunlight that have nothing to do with vitamin D.
Does a sunroom or a greenhouse give you vitamin D?
No. A sunroom, a greenhouse, and a conservatory are all glass, and glass is the thing that removes the UV-B your skin needs. The room can be hot and dazzling and still contribute nothing, because heat is infrared and dazzle is visible light. Neither of those is the band vitamin D is made from.
Does glass block UV?
Glass blocks the UV-B wavelengths that vitamin D synthesis depends on, roughly 290 to 320 nanometres. That a pane lets through plenty of visible light and warmth tells you nothing about UV-B, which is why a window can feel like strong sun while contributing nothing to vitamin D.
If the sun feels strong through the glass, why does that not count?
The heat is infrared and the dazzle is visible light, and both pass through a pane easily while carrying no information about vitamin D. Warmth and brightness are exactly the two cues that feel like evidence and are not. Vitamin D is made from ultraviolet B alone, roughly 290 to 320 nanometres, and that is the one band the glass removes.
Does the time of day or the season change what a window gives you?
No. Behind glass the answer stays zero in June at noon and in December at dusk, because the pane removes the band regardless of how high the sun has climbed. Sun height genuinely matters, but only outdoors, where a high sun is what lets UV-B reach the ground in the first place.
Can you get a tan or sunburn through a window?
Tanning and burning are a different question from vitamin D, and the vitamin D answer does not settle them. What is firm is that glass blocks UV-B, so sunlight through a window makes no vitamin D in skin. That does not establish that a given pane blocks every ultraviolet wavelength, so do not read no vitamin D as proof of no ultraviolet exposure.
Does sitting by a sunny window help at all?
Not for vitamin D, though it is not nothing for brightness. Bhandary and colleagues measured eye level at a few hundred lux in rooms with windows against around 9,300 in the open, so the window seat is the brightest chair in the house while staying far below the door. For vitamin D specifically it contributes zero, because the UV-B never arrives.
Do you need bare skin, or does light through clothing count?
Vitamin D synthesis needs ultraviolet B to land on skin, so sunlight falling on covered skin is not doing it. That is not an argument for uncovering more. Ultraviolet radiation is a carcinogen and the most preventable cause of skin cancer, and covering skin is one of the protections called for once the UV Index reaches 3, alongside shade and correctly used sunscreen, which costs you no synthesis.
Is a UV lamp a substitute for sunlight for vitamin D?
Do not use a UV lamp as a vitamin D substitute. Vitamin D synthesis requires UV-B, but ultraviolet radiation is a carcinogen and the most preventable cause of skin cancer. That mechanism is not a reason to buy a lamp. Sunlight in moderation, taken in the gentler hours the UV Index shows you, is a different decision from putting an ultraviolet source on your desk.
Sources cited on this page
Every number on this guide page traces to one of the 5 sources below: 3 peer-reviewed, 1 government or regulatory.
Peer-reviewed
- Bogh et al., 2016 (Photochem Photobiol Sci; major inter-personal variation in the 25-hydroxyvitamin D increase induced by the same UV-B dose)
- Young et al., 2019 (British Journal of Dermatology; correct sunscreen use and vitamin D status)
- Bhandary et al., 2021 (PLOS ONE; measured eye-level light levels across indoor and outdoor locations)
Government & regulatory
Institutional & standards
- WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide
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