Can I make vitamin D from the sun right now?

You can make vitamin D right now only if the UV index is 3 or higher and the sun is high in the sky — roughly more than 45 degrees above the horizon. A quick test: if your shadow on the ground is shorter than you are tall, the sun is high enough and you can make vitamin D. If your shadow is longer than your height, you are making little or none, no matter how bright it looks.

Can I make vitamin D in winter?

Above roughly 35 degrees latitude, the sun stays too low through the winter months for your skin to make meaningful vitamin D, even at midday on a clear day. This is called the vitamin D winter. The further from the equator you live, the longer it lasts — from October to March in cities around 40 degrees N, and longer still further north.

Can I make vitamin D through a window?

No. Ordinary glass blocks virtually all UVB, the wavelength your skin needs to make vitamin D. Sitting in a sunny window gives you no vitamin D — only UVA, which still ages skin. You need direct outdoor sunlight on bare skin.

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Can I Make Vitamin D Right Now?

Short answer: only if the UV index is 3 or higher and the sun is high in the sky — roughly more than 45° above the horizon.

Ten-second test: look at your shadow in direct sun. If it's shorter than you are tall, the sun is high enough to make vitamin D. If your shadow is longer than your height, you're making little or none — even if it feels bright and warm.

Vitamin D is made when a specific slice of sunlight — UVB, wavelengths of 290–315 nm — hits bare skin and converts the cholesterol-like molecule 7-dehydrocholesterol into vitamin D3. The catch is that UVB is fragile: it's absorbed strongly by the ozone layer, and when the sun is low, sunlight takes a long, shallow path through the atmosphere and almost all the UVB is filtered out before it reaches the ground. Visible light and warmth get through, so it can feel like great sun while producing no vitamin D at all.

That's why "right now" is the right question. The answer changes through the day, with the season, and with where you are. Here's how to check.

The two things that have to be true

1. The UV index is 3 or higher

Below a UV index of about 3, vitamin D synthesis is negligible no matter how long you stay out. At UV 3+ meaningful production begins, and it speeds up as the index climbs. Most weather apps show the current UV index; you want it at 3 or above. (UV 3+ is also the level at which sun protection starts to matter for fair skin — the same sun that makes vitamin D can burn you, so the two go together.)

2. The sun is high enough — the shadow rule

UV index alone isn't the whole story, because the same index can mean different amounts of the vitamin-D-specific UVB depending on the sun's angle. The lower the sun, the longer the atmospheric path and the more UVB is stripped out. As a rule of thumb, you need the sun roughly 45° or higher above the horizon.

You don't need an instrument for this — use your shadow:

Shadow shorter than your height → sun is high, UVB is getting through, vitamin D is possible.
Shadow about equal to your height → the sun is near 45°, the borderline.
Shadow longer than your height → sun is too low; little to no vitamin D, regardless of how bright it is.

This is why midday — roughly 10 a.m. to 3 p.m. — is the window that matters. Early morning and late afternoon sun is lovely and low-risk, but it's also low-UVB.

When the answer is "no" no matter the time of day

Winter at higher latitudes

If you live more than about 35° from the equator, there's a stretch of winter when the sun never climbs high enough at any point in the day. This is the well-documented "vitamin D winter": at these latitudes the midday sun in December is simply too low. Around 40°N (Madrid, New York, Beijing) it runs roughly October–March; the further north you go, the longer it lasts. Boston and Edmonton are the classic study examples where winter sun produced no measurable vitamin D3 in skin.

Through glass

Ordinary window glass blocks virtually all UVB. Sitting in a sunny window — at home, in the office, or in a car — gives you zero vitamin D. (It still lets UVA through, which ages skin, so it's the worst of both.) You need direct outdoor sun on bare skin.

Other things that quietly switch it off

Heavy cloud or smog — clouds don't block UV completely (a fully overcast sky still passes roughly 40%), but thick cover plus a low sun can push you below the threshold.
Covered skin — UVB only works on skin it actually reaches. Fully clothed means no synthesis.
Sunscreen — in lab conditions SPF 30 blocks ~97% of UVB. In real life people apply it thinly, so it blocks less than the label suggests, but it does reduce production.

The honest part: "possible" isn't "how much"

Passing the shadow-and-UV check tells you the window is open — not how much vitamin D you'll actually make. That depends on your skin type, how much skin is bare, your age, and how long you stay out. A pale-skinned person at UV 6 might get a useful dose in 10–15 minutes; someone with very dark skin needs considerably longer for the same amount. If you want the underlying maths, see how a sun exposure calculator works.

Get a real-time yes/no for exactly where you are

Healthy Weather reads the live UV index and computes the sun's angle for your location, then tells you whether your vitamin D window is open right now — and roughly how long to stay out. No shadow-guessing. Free to try on iPhone.

This guide is general information, not medical advice. Healthy Weather is a planning tool, not a medical device, and can't account for your personal health, medications, or skin. The only reliable measure of your vitamin D status is a blood test ordered by a doctor. See our FAQ and Terms.